There appears to be a bug when increasing maximum HP per level when levelling up. There is a minimum +1 per level to your Max HP. 

2014: https://www.dndbeyond.com/sources/dnd/basic-rules-2014/step-by-step-characters#ClassFeaturesandHitDice

Each time you gain a level, you gain 1 additional Hit Die. Roll that Hit Die, add your Constitution modifier to the roll, and add the total (minimum of 1) to your hit point maximum. Alternatively, you can use the fixed value shown in your class entry, which is the average result of the die roll (rounded up).

2024: https://www.dndbeyond.com/sources/dnd/free-rules/creating-a-character#GainingaLevel

Adjust Hit Points and Hit Point Dice. Each time you gain a level, you gain an additional Hit Die. Roll that die, add your Constitution modifier to the roll, and add the total (minimum of 1) to your Hit Point maximum. Instead of rolling, you can use the fixed value shown in the Fixed Hit Points by Class table.

So this means even if you have a Con score of 1 (-5 modifier), and you were a Wizard (d6 die, average / fixed: 4), you should always be increasing your Max HP by 1 each level as a minimum, even if you rolled for hit dice and got a 1. 

However, when testing this on a character (Wizard, con score of 1), there is no minimum +1 increase per level and instead just has an overall total minimum of 1. For example, I used rolling, and put 8 (starting as 6 for 1st level and rolling 1 on 2nd and 3rd level) but with a -5 per level con mod, -15 overall, this is set as 1 instead. It should be 3 minimum. 

The minimum Max HP is equal to their level - the minimum you can start with is 1 and you gain +1 minimum per level. This should be reflected in the character builder, but it isn't.

Basically level 1 you get 10 + con modifier. At level 2 you get to chose (1d10 + con modifier) or (6 + con modifier). The average for 1d10 is 5.5. So 6 is on average better.

For your question I would chose 6 + con modifier.

You had 10+2 at level 1(12 max). You got 6+2 at level 2(20 max). Chose 6+2 at level 3 and have 28 max hp.

Quote from harvey936 >>

Basically level 1 you get 10 + con modifier. At level 2 you get to chose (1d10 + con modifier) or (6 + con modifier). The average for 1d10 is 5.5. So 6 is on average better.

For your question I would chose 6 + con modifier.

You had 10+2 at level 1(12 max). You got 6+2 at level 2(20 max). Chose 6+2 at level 3 and have 28 max hp.

 
Is this a bot comment? Or just not reading my post properly?

What does this have to do with the min +1 hp per level rule not being implemented properly by the character builder?