As my picture shows, I had Fii as enemy in the endless dungeon, and this healed Nana in Phoenix form.
In case you haven't noticed that it happens there as well, but the portrait of Nana when loading a file still shows the last form she took, even if the save applies to a different form (like when having a bad end). This doesn't persist if you actually shut down the game. Not sure if your fix applies here, too.
I believe that transforming around with "TF items automatically assigned" and having Fuwa Family Plush as reserve seems to interact a bit strangely. Most likely the extra hp from the item get lost, but not the max hp. Shouldn't such items allow for actual hp when being equipped?