There seems to be a problem with an immortal dragon.
This can happen when the dragon hits zero HP while she is flying, which can be caused for example by mark of death, there may be other ways as well.
If she is in the air, hits zero HP, then she tries to do 2 animations at once: Landing and Dying.
Which results in an immortal dragon that is flying actually halfway BENEATH the ground of the arena.
(i.e. half her body is below the ground)
Had something somewhat related to this tonight.
PC, Perilous, Frost Dragon. Enemies beforehand were demons if it matters.
During the zone 5 dragon battle, it eventually took off with a very small sliver of health, then landed on one of the rooftops. My friend launched the trebuchet at it, which hit it and took away the remaining health bar. However, the dragon just sat there on top of the roof doing nothing. It could not be targeted and didn't leave the roof.
Host migration to reset the zone allowed us to beat the dragon again, and this time it was killed on the ground and the mission proceeded to the completion screen normally.