I'm working on this right now.
Generally the problem is that chunks are queued for saving and then immediately unloaded, before they are saved; then the save thread asks for the chunk data and finds out there's no chunk, so it silently bails out
Generally the problem is that chunks are queued for saving and then immediately unloaded, before they are saved; then the save thread asks for the chunk data and finds out there's no chunk, so it silently bails out