From licensee:
"Testing incremental GC on our project we have ran into an issue with level streaming where levels will get stuck in the 'loading' state in packaged builds. The issue appears to be due to the fact that a level's package/world will get marked Garbage at some point (probably as part of streaming out), but then LevelStreaming will want the level to be loaded and start requesting the level's package to be loaded each frame; The act of requesting the level's package for loading causes the garbage package in memory to be marked as reachable (MarkAsReachable), meaning it won't collect- because it gets marked every frame as long as your incremental GC takes multiple frames it will never be collected (even though it's garbage) because it will always be marked reachable."