What it says on the tin. First of all, bShouldBlockOnLoad is broken, which forces me to flush the level streaming system before I proceed. Band-aids on band-aids.
But the real issue is that my actors are not getting BeginPlay() called. I noticed this because nothing was initting or ticking. When I finally allowed the parent blueprint of one actor in question to tick before BeginPlay was called, it worked. In other words, BeginPlay is simply not getting called.
This is 100% an issue with level streaming. BeginPlay() and Tick() are absolutely called when the level is run standalone as opposed to streamed in. If I stream the level in without flushing, it also gets called. Unreal 5.1.
I found other issues in the same vein from some years back, which claim to be fixed.