I'm currently running into an issue with the homebrew system. I'm creating an option for a subrace that overrides the unarmored armor class to be 12 + DEX. However, when I use "Unarmored Armor Class" with both the set and bonus modifier types, it treats it as bonus. When I try using "armor class" with the set modifier type, it reverts to the default 10 + DEX.

Try Set -> minimum base armor --> fixed value --> 12

That works as a stop-gap solution, but it won't be an effective option 100% of the time. If the character's DEX modifier is negative, it should still dip below 12, but using Set Minimum Base Armor prevents that from happening

Create a copy of a race (or subrace) with natural armor to see how they do it.

Quote from Stormknight >>

Here's how the modifier is set for Lizardfolk, one of the official races:

 Setting it up this way did in fact work, but I still find it odd that bonus and set are the same functionality. The way I'd expect it to work is "Set Unarmored Armor Class" with a value of 12 to be the same as "Bonus Unarmored Armor Class" with a value of 2. Thanks for the workaround!

I've been reading multiple threads on this and trying to follow exactly what everyone is doing here, but I'm still somehow getting a +3 for unarmored defense bonus on my character sheet at 2nd level. I then try to have a feature at 10th level that replaces this 2nd level feature with a '5' in the fixed value. Which according to this thread should give the character a 15 + DEX. When i level the character up to 10th level, I'm now getting the unarmored to be +2.