I definitely understand that. Pointless irritations for no other purpose than to waste a few more precious seconds. This damned game is littered with them.
Always doing the laziest possible fixes. I assume it went down like so. Master levels are carrying over when level capped, so if we just cap the pets, after the battle they'll be uncapped and the next entrance it'll be rebroken so instead of FIXING the problem, they said just despawn ALL pets. and as a side effect, they did it to ALL instances.
There was one small issue, automatons in level capped bc's were broken (maybe bst pets), so they made all pets in all instances despawn as a fix. Cause it's less effort to do that than actually figure out why and address it.