Copying this from the US forums, #Blizzard, we need fix!!!
''As the title suggests, the following is a rather annoying, presumably unintended, interaction I’ve run into on the WotLK prepatch PTR:
When an unholy DK with an actively summoned Ghoul (talented via Night of the Dead) leaves the ground on a flying mount, the Ghoul completely despawns, requiring the DK to completely resummon the pet, costing a cooldown, as well as a corpse dust/enemy corpse if unglyphed. This is INCREDIBLY annoying to play around, and despite me reporting it in the last 3 beta builds, it has still managed to slip its way into the version currently on PTR.
I can confirm that this glitch is NOT the case for the non-permanent ghoul summoned by an untalented use of Raise Dead. He will run around below you on the ground for his fixed duration of 1 minute. This behaviour is intended, whereas the behaviour of the version talented via Night of the Dead should not be the case. I also tested on a hunter, and though the pet despawns when I fly up, it instantly respawns upon dismounting when I return to the ground.’’ - https://us.forums.blizzard.com/en/wow/t/annoying-dk-ghoul-bug-going-live-with-prepatch-today/1320414