Not sure which is the bug and which is the ‘working as intended,’ so here goes:
Unholy DK with the talent Master of Ghouls which makes the Ghoul a pet under the control of the DK will initially have the ‘pet’ de-spawn once the DK takes to flight.
Now, if the DK has the pet summoned and then logs out inside and inn and then logs back in, the pet no longer de-spawns when taking flight.
This changes if you do any action like a quest that puts you into story mode as an example, but log out in an inn and pet wont de-spawn.
I just bought flying mount and when I took to the air my ghoul pet disapeared completely and after I land I need to resummon him with raise dead skill…
Summon a pet, wait for the animation to end, wait an additional 5-10 seconds, close the game without mounting, wait 1-2 minutes until your character completely disconnects, relog, you should have your pet out, mount up and fly, go back to the ground and the pet should respawn.