I have this exact setup ( your setup before modifying the GI ).
I just tried your ‘open level’ approach, and indeed, the streaming is flaky.
I’m wondering, is you game mode the same in all levels? ( mine is ).
Tip ( if you haven’t done this ). All my level instances have no lighting, which is a pain during editing. In fact, you can include the persistent as a sublevel to each level, so you can see what you’re doing. It doesn’t get loaded by ‘load level instance’.
I know that’s a bit off-topic, but I found it handy for this setup.
Update: Very weird, I tried it again, and now I can’t make it go wrong. It seems good enough for testing, although I wouldn’t use it in a release…