Looks like they would not have been removed if they disconnected during the chunk load, preventing the world from being unloaded.
Logging into an unloaded world isn't going to end well. This may fix the cases of people seeing errors about regionfiles being closed, as loading chunks in an unloaded world will cause this as the regionfile cache is closed but not cleared.
We cannot put blocking network I/O onto the worldgen threads, this will crash the server if it stalls