When changing an instances parent, if the instance has a “Bone” as a descendant, no matter how far of a descendant it is, then the instance will not show the children drop down when the parent is changed.
If you undo this changing parent action, the instance is still bugged
The only way i have found to fix this is to add a random instance to the bugged instance using the plus sign, and then deleting that added instance,
Restarting studio will un-bug and bugged instance, but does not stop the situation from occurring
This seems to happen in any game.