Reposting from another thread for awareness:
I've seen this happen multiple times to my friend who mains Yamato.
We did some testing in sandbox mode and came to a few conclusions.
It seems that Djinn's Mark is coded as a debuff of some sort, and no matter how its removed - Mirage activating Djinn's Mark, timeout, debuff remover or Yamato Ultimate - causes the Mark to go off and deal damage.
Specifically in the case of Yamato's Ultimate things seems to trigger in an order that causes the damage to hit before Yamato gains any immunities.
By description Shadow Transformation says "After an initial invincible transformation... and you gain immunity to negative status effects." This should imply the immunity activates before removing any negative status effects, such as Djinn's Mark.
Some clarity on how Djinn's Mark should works would be nice. Should the damage happen even if its cleansed or removed? Or should removing it from any means other than timeout or Mirage activating the ability cause it to just fizzle and do no damage.
Obviously this game is in early Alpha and there are a million interactions happening every game, but this one is a specific one that is, I don't want to say game breaking.. But very very annoying and difficult to deal with in the Yamato vs. Mirage matchup.