I have been leveling my Black Mage, and I'm extremely confused by Umbral Hearts, I recently unlocked Despair, and assumed it'd interact with Umbral Heart the same way Flare would. After it didn't I went to go look for a reason why, thinking it would be a future trait or something.
But what it lead me to realize, is that it's not explained anywhere (at least that I can find) on why or how it works with Flare! The Umbral Heart trait doesn't explain it, the Job Gauge description doesn't mention it, and Flares tooltip doesn't talk about it either. I tried to look it up and found there used to be an Enhanced Umbral Heart trait that explained it, but was since removed. It's very confusing! It really should be mentioned somewhere. I think it's important to explain, especially considering how it works differently then the normal use of Umbral Heart.
Please let me if there is an explanation in game.