Actually, this happened couple days ago when I cleared Tower of Abyss, where my Rachel got killed on her Phoenix mode.

Then proceeded to do Easy mode Iron Devourer.

My Rachel spawned already in Phoenix mode and has no more Phoenix revive when she was killed.

Just a heads up for everyone, or this can be treated as bug report.

EDIT: Failed -> Cleared

Ah, I forgot. Before entering Iron Devourer, I re-checked again that my Rachel can still re-enter the next day requirement, despite already being used for that day. This denote the Rachel displayed on the heroes list is Phoenix mode Rachel, not normal mode.

Also, after finishing Iron Devourer battle, my Rachel turned back into normal mode again. Re-checking the formation for the next day battle showed me Rachel being crossed-out.