The ghost does not respawn when you dismount from your flying mount. Normal pets have been fixed with today’s patch, they respawn on land but somehow this memento pet didn’t.
Please address this issue since now you have to put the memento in the bank and back into you bag to make the ghost spawn again.
The haunted memento ghost is permanent. You probably have the quest item from the cure quest in Shattrath in your bags which causes it to disappear and reappear and wander a little for some reason, you also get whispers from it.
edit: this is a reply to the above comment, as for OP i have no idea but i’ll take your word for it that it’s bugged upon dismounting.
edit2: your character should do the /scared emote whenever it spawns, i remember seeing that a lot everytime i dismounted in original wotlk or this might’ve been added later i’m high af