This partially works, when building the whole level (Build All Levels), but the same thing still occurs with static lights. Static lighting does not seems to be supported under World Partition (I can reason why not), so I suppose World Partition fixes the disappearing actors problem, but static lighting is still not feasible in UE 5.1 in any way. With traditional worlds the actors disappear on static lighting build, and with world partition it’s not even supported.