OK, it may be a bug, but it could be a loading range issue, I can’t say if this will be a waste of your time or not. But I’m worried the loading range and loaded cells are outside the auto cull distance.
Thanks for confirming that changing view distance does not affect the auto cull, I never tested it myself, only assumed.
In world settings, turn on grid visibility.
Go up high over 0,0,0 and look down, your loaded cells are white below you, adjust your cell size and loading range, so no walls have loaded cells touching them.
You can go a large cell size, almost to the boundary walls with a loading range smaller than the cell size, if you have plenty of memory available to use.
If your running low on memory, you can use a much smaller cell size, with a loading range that does not cause any cells that have the boundary walls in them loaded.
Build HLODs and test, hopefully it shows the HLOD mesh