Weird fly height problems

So you can use the fly trick and add/remove the crow form ability from a unit, and set its fly height.

Sometimes in the game, some units just stop flying. It usually happens on heroes. Any idea what could cause this?

Morphing skills are involved?.

I would say that it is better to always add/remove the ability before changing the flying height

Hm I do that. I just found out that entangling roots buffs prevents any flying.