One of my characters has the Entertainer background and when choosing a flaw I saw 'Despite my best efforts, I am unreliable to my friends.' and I thought that kind of fits my character - but was wondering how you would go about role-playing that subtly?

My character is the kind of person who when things look bad she's likely to run away - but I wouldn't want to do that to the group very often - so what other low-key ways would you show it if you tried that flaw? Just looking for general ideas.

She also has the personality trait of 'I lie about almost everything, even when there's no good reason to.' - if that influences any answer.

Without being labeled the coward of the party, you could just be that person who likes to wander off and do their own thing for a bit.  Maybe you can make a big deal about misplacing some party valuable that you took off the BBEG. 

Is your character good a lying or bad at it?  Being a bad liar, but liking to lie anyway is, by itself, likely to make your character quite unpopular.  However, being good at lying makes you questionable, but still useful to have around.

You seem like the kind of character who would take group loot and bet it on the ponies?  Then steal from them so you could win it all back in a poker game and make it up to them (with disastrous results)?

The two link together, really.  If you lie about everything, that makes you pretty unreliable.

One option is to simply flake on your out-of-combat responsibilities.  Your turn on watch?  Nod off.  Get distracted watching the prisoner.  That sort of thing.

That's actually a really tough flaw to play well, because it goes against many of the out-of-character assumptions people will make. If people expect to play cooperatively and you play like a bad teammate - abandoning your team, messing up their plans, etc - people will feel like you personally (not your character, YOU) are sabotaging their gameplay experience. Same with splitting the party - if you're wandering around doing your own thing, you've got a chance of coming across to the GM and other tablemates like a "special snowflake" player (making the GM prepare one adventure for the group, and a second solo adventure for you as you wander off)

Both that one and the lying flaw probably have to be played with a very light touch to make it fun and not annoying. Don't be like the guy in this post https://www.dndbeyond.com/forums/d-d-beyond-general/general-discussion/42271-about-dms-and-players-radicalisms .

I would say one thing you could do is pick a very non-supportive build. Don't take healing spells, or auras that buff allies, etc. In combat play as best you can with the tools you have available, but when there's a level-up choice of what to go for, pick the DPS option. This will make your character play in-combat as a lone-wolf attacker type, while not being blatantly anti-party like "having healing but avoiding using it".

For the lying, I would recommend mostly playing that as being fairly distrustful of NPCs, and preferring to mislead them about who you are and why you're there rather than being honest and asking for help. I'd recommend NOT treating that as a reason to lie to your own party all the time.

Oh she's a good liar. She's proficient in Deception and +3 Charisma at level 1 and I plan on multiclassing into Rogue (from Sorcerer) to get expertise in it - unless there's an easier way.

She's a Changeling who's had about 10 different masks in her backstory. Usually cases of her thinking people have found out she's a Changeling and running away to become a new person - or setting a building on fire by accident with magic.
She's not greedy or a gambler or anything like that. Personal wealth means little to her. She basically lives to play music in taverns and inns and would love nothing more than to just settle down - but for too complicated a reason to go into - she's become a Divine Soul Sorcerer and her errant magic (being tested by a deity) keeps preventing that from happening. The deity wants her to face her past but she refuses (for now). Basically she's pissed off and the lying thing is partly a middle finger to the deity and also because she doesn't want to face the truth. She's a bit of a gossip too.

But you don't have to suggest anything to specifically fit my character. Just any general idea what you might do with that flaw if you had it would at least give me some ideas to play with.

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For the lying, I would recommend mostly playing that as being fairly distrustful of NPCs, and preferring to mislead them about who you are and why you're there rather than being honest and asking for help. I'd recommend NOT treating that as a reason to lie to your own party all the time.

That's a very good point. Yea I plan on it being mostly lying to NPC's. Maybe the party at first about who my character is but they'll quickly realise what's up and the truth will out.

And yea - being annoying is something I hope to avoid - so anything subtle would be key.

Might I suggest a foil to your character's roleplaying? In the group I play in, a player has a character much like yours might be. Young, brash, unreliable, causes alot of collateral damage. It was originally suggested to change the alignment from chaotic good to chaotic neutral but, the character had an extreme soft spot for children, animals, the poor, just causes etc. We let the good alignment stand. Now my cleric is like a big brother, mentor and trusted friend. I can temper some of her chaotic nature and smooth over problems with party members or NPCs. This adds a greater amount of group roleplaying vs a single player's character. 

There are a series of Warhammer 40k books about a guy named Commissar Ciaphas Cain.  His thing is that he is a cowered to the extreme but he also doesn't like screwing people over.  He accidently by shear bad luck keeps on finding himself in these extremely dangerous and deadly situations where people are counting on him or he just ends up being in the right place at the right time.  For example he ran when his ship was destroyed into a escape pod, then happened across the remnants of a devastating defeat by Orks who, for his own self preservation, he turned into a convoy that slowly grew and when he caught up to the front lines he just happened to stubble upon the Ork Warboss and have to duel him.  He would fight desperately to survive, win, and become a hero because everyone thought he planned it.  He's just trying to survive and get out of the dangerous situation but ends up neck deep in it time and time again.  I think that's a way to play him.  In other words he messes up but in ways that help over all.  Yea he forgot the arrows but it just so happens he trips over a rock that has 60 magic arrows under it.  That sort of thing.

I'd say that in combat the important thing is to prioritize your own survival over the survival of your friends. Like... let's say you're out of healing spell slots and you've got a buddy low on health with someone big and nasty in front of them... you could run over, give them a potion, and put yourself in danger in the process... or you could just keep your distance, maybe fire off your crossbow or maybe just focus on stealth when things get hairy.

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There are a series of Warhammer 40k books about a guy named Commissar Ciaphas Cain.  His thing is that he is a cowered to the extreme but he also doesn't like screwing people over.  He accidently by shear bad luck keeps on finding himself in these extremely dangerous and deadly situations where people are counting on him or he just ends up being in the right place at the right time.  For example he ran when his ship was destroyed into a escape pod, then happened across the remnants of a devastating defeat by Orks who, for his own self preservation, he turned into a convoy that slowly grew and when he caught up to the front lines he just happened to stubble upon the Ork Warboss and have to duel him.  He would fight desperately to survive, win, and become a hero because everyone thought he planned it.  He's just trying to survive and get out of the dangerous situation but ends up neck deep in it time and time again.  I think that's a way to play him.  In other words he messes up but in ways that help over all.  Yea he forgot the arrows but it just so happens he trips over a rock that has 60 magic arrows under it.  That sort of thing.

 The accidental hero idea sounds great when the DM is fully onboard. I like it...a lot.