I have a level 20 wizard using fixed HP that has a Con of 3. The PHB says:

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).

At level 1, I have 2 HP, which is correct. Every level after that, I gain 4 HP from Wizard and subtract 4 from it due to my Con modifier. But because of the above rule, the amount I add each level is 1, as that is the minimum. Therefore, at level 20, my wizard should have 21 HP. Instead, mine says 2. It doesn't seem like it's including a minimum each level.

Character in question: https://ddb.ac/characters/102576079/HZklg7

Check in the HP section in the builder that you don't have anything in any of the text boxes, especially the 'override Max HP' section.

Both HP Modifier and Override HP boxes are empty. It says Fixed HP 82, and under Hit Point Bonuses it says -80 from Constitution. It's calculating this character's HP at 2 with that, but it's not supposed to.

Gotcha. I think, if I remember correctly, that the (minimum 1) was technically errata'd in, and they fixed the text, but I thought they had fixed the HP calculator. Either I misremembered and they hadn't, or something more recent undid the change.

Since the 21 HP is the correct amount you should have, best suggestion I have until the issue is fixed is to use the Override max HP box to set your correct HP, and make a note of it somewhere, in case you also have to use the box due to some magical effect that temporarily lowers your max HP, so you can reset it to the correct amount.