To my knowledge World Partition doesn’t work the same as World Composition with the sublevels etc. Instead it uses an array of grids (they can be different cell sizes and streaming distances) that you assign actors to. That’s why you assign a grid to the actor.
However, it seems like World Partition doesn’t notice what grid the spawned actor is on despite the settings given to it. I don’t know if there is some sort of “hey register this new actor to your grid” or something system… but yeah doesn’t work.