I think i figured out this weird bug.
If you mount up with any repair mount and zone in. Agatha will despawn along with those passenger from the mount. I started just unmounting when i zone in and she hasnt despawned since.
edit: did 4 zoning with a repair mount and all 4 times she despawns. So yup it has something to do with her despawning with the mount passengers
Make sure it is only you entering. No pets. No merchants from the mount. That is it.
Disabling the addon PetLeash (keeps non-combat pet summoned) and manually dismissing my pet made Agatha stop despawning on my Fury warrior. Haven’t had this issue on any other challenge with non-combat pets being summoned.
I had this happen on my Unholy DK as well. Many of the posts here say dismissing your non combat pets will work but I did not have any so I dismissed my combat “pets” (ghoul and skeleton) then entered and it was fine. I did not die to reset the instance, I just clicked to leave it like you would at the end of a normal dungeon or raid.
She is still despawning seconds after entering the encounter. Guess no Mage Tower for me right now.
this worked for me
ty