Tortle Monk AC

I created a Tortle (AC 17) monk (unarmored defense)

After leveling with some attribute increases (20 dex and 20 wis):
Tortle Natural Armor: 17
Unarmored Defense: 10 + 5 (dex mod) + 5 (wis mod) = 20

Shouldn't the higher be used?

Character sheet is showing 17. 

Is this a bug or am I misunderstanding?

If you click on AC, are there any overrides or anything? 

Just the normal overrides. I can set it to 20 with an override.

I assumed DnDBeyond follows the rules, but I guess there are some edge cases they haven't accounted for.

I’ve double checked and got the same error, for Legacy and updated Tortles. Unfortunate! But at least not gamebreaking. Strange it doesn’t do it in this case though, it probably should. 

Actually I think character sheet is correct, strictly according to RAW. If we read the Unarmored Defense feature:

Unarmored Defense

Beginning at 1st level, while you are wearing no armor and not wielding a shield, your AC equals 10 + your Dexterity modifier + your Wisdom modifier.

As highlighted, it stats when not wearing armor, you use the 10 + your Dexterity modifier + your Wisdom modifier AC calculation. However, when we read the Tortle descriptions:

Natural Armor

Your shell provides you a base AC of 17 (your Dexterity modifier doesn’t affect this number). You can’t wear light, medium, or heavy armor, but if you are using a shield, you can apply the shield’s bonus as normal.

Basically your shell is still armor, it's even in the name of the ability itself. However, I personally would still let a Tortle Monk player use the Unarmored Defense feature regardless. Also it's interesting to not that it still seems to be adding move speed increases from Unarmored Movement.

nah, I solved the mystery: dexterity does not affect their AC. to demonstrate this, manually change wisdom to 26. you'll notice AC goes up to 18 from its +8 modifier. this is RAW, not a sheet bug... :\

So it's a coding issue. The Tortle's AC is set in the back end by setting their DEX AC bonus to 0, then adding 7 to the current defense (10+7=17). The WIS score is still being added, but even if you have 20 WIS, your AC will only be set to 15 (10+5(WIS)+0(DEX)).  I set up a character and made his WIS score 30 to test it, and his AC did change to the higher 20. So I fixed it with a homebrew race (Dragon Tortle). In the modifier's screen, select the "Modifier Type" to be "SET", then select "Modifier Subtype" as "Minimum Base Armor". Finally, change the "Fixed Value" to 17 or whatever you want. If you're editing the Tortle to create this, you also need to delete the other modifiers that are already there.