I have created a couple feats that require The Fiend as a class prerequisite. Unfortunately when I go to the character sheets to add these feats the feat is listed as Unavailable because Prerequisites Not Met. I have tried this with other subclasses and on different character sheets but I still run into this problem. To me it seems like having a subclass as a prerequisite, even though they're selectable options, are bugged and don't actually determine if a sublcass is active or not. I know I can add these anyways but I figured it makes it more inconvenient for others who might add it to their homebrew collections. 

Example The Fiend feat: https://www.dndbeyond.com/feats/1197966-infernal-tactics

Since none of the official feats have subclasses as prerequisites, that part of the coding was never finished.

Fascinating. Will go ahead and just switch the prerequisite to Warlock for now.

Any chance this feature will be finished? I have some ideas of subclass-specific feats and it’s a bummer to hear that they put out a UI with no backend. 

Pro’ly not, no.