I’m also experiencing this problem; I create a new empty open world map, add a few actors at varying distances, save it, close the engine, reopen the project, open the level, and only the closest actors are available. The others are unloaded. I try to open the World Partition panel (which has mysteriously closed since I restarted the engine) and my RAM immediately maxes out, stalling my system so hard it either freezes and I have to reboot, or I somehow managed to kill UE and my system recovers.
I have yet to find a fix for this. I tried to build HLODs, but that also maxed out my memory before crashing UE.