I'm looking at this stat block from the Monsters -> Official tab (and after clicking on the name to view it as a stand alone tab), and the damage resistance is missing. There should be "Damage Resistance damage from spells; nonmagical bludgeoning, piercing, and slashing (from stoneskin)" as seen in the monster manual, but I don't se this. If a possible DM looked here instead of the MM they wouldn't get the full monster. (as seen on 8/11/2018)

Good find, I see that too. Just checked the SRD v5.1 and it reads

Damage Resistance damage from spells; nonmagical bludgeoning, piercing, and slashing (from stoneskin)

*The archmage casts mind blank, stoneskin and mage armor spells on itself before combat.*

They covered that in this post script after the spell list. They aren't inherent resistances just magically imposed resistances from spells cast before combat. Surprise attacks Still have to have a chance against mages.

In the monster manual, you will find "damage from spells" under resistances, where as you will not see that here. That's a big deal to have. That's because technically the Archmage is a wizard of abjuration (minus the temp HP from the lv 2 feature).

Yeah, seems they copped out on that specific resistance and just have them advantage on magic saving throws instead. 

Funny story, our level six party tried to rob a magic shop, but we didn't know the owner was an arch mage.

That didn't go very well.

Quote from DIemortalscum >>

Funny story, our level six party tried to rob a magic shop, but we didn't know the owner was an arch mage.

That didn't go very well.

To be fair, we’ve all made this mistake.

Why does an 18th level spellcaster with a +5 ability modifier have +9 to spell attacks and a DC 17 spell save? Shouldn't it be +11 and DC 19? (apologies if my math is wrong, but at level 18 the proficiency bonus for a full caster is +6). I've noticed the same math on the Archdruid, so maybe I'm mistaken or missing an important rule.

Monster's have different rules for their proficiency bonus. Instead of it being based off of their level, it's based off of their CR. So as a CR 12 monster, the Archmage has a proficiency bonus of +4. If it was a CR 13 monster, the Archmage would have a +5 proficiency bonus. That's also why the hit die size is a d8 instead of a d6, because the Archmage is a medium creature.

Thank you for the clarification!

I am making a homebrew Matt Mercer, and he has the casting ability of an Archmage.

No shield? No fireball?

If the resistances and stat bonuses from an ‘already cast’ mage armor and stoneskin are included in their stat block you would think the same would apply with mind blank—which is also ‘already cast’ and has a longer duration without concentration.

Specifically it should include:

Damage Immunities: psychic (from mind blank)

Condition Immunities: charmed (from mind blank)

I know this is an issue originating with the source material and not dndbeyond, however we cannot leave comments in other people’s sourcebooks.

I really like this monster and I used it for my last campaign and it was awesome hopefully you make more awesome creatures.

-Thank you 

DiceyDM. Why is the outline of the thing you said outlined in red? How did you do it?

Pretty sure he's a moderator on DnD Beyond, @Theren_Melimamne

sounds like a certain marisha ray did the voice on this one

So if this creature has resistance to " Damage from Spells; Nonmagical Bludgeoning, Piercing, and Slashing (from Stoneskin)"   Isn't that basically resistance to all damage?     

Quote from tjfuquay >>

So if this creature has resistance to " Damage from Spells; Nonmagical Bludgeoning, Piercing, and Slashing (from Stoneskin)"   Isn't that basically resistance to all damage?     

To give you some examples of damage that wouldn't be resisted by the Archmage.
Any dragon's breath weapon, any natural poison from a monster, natural hazards like lava etc.