@yakuko: the most probable answer is: when you shift to the crow you spend mana, so you're in "regen" until your mana is full. If HUD is still visible AFTER you have 100% of your mana - let me know please.
That's the thing, we can herb in flight form, so there is zero shifting. That was just one example. I've also had it stay showing even after drinking and I can see both my health and mana are full, yet the hud won't go away. Or sometimes it won't even show when I am regening, it'll stay transparent.