When a file is removed while it is still being loaded (for example by removing it, or clearing the session, right after adding files), its load callback is never called:
  File._loading_finished returns early when the file is no longer pending.

That callback is what Tagger._file_loaded uses to decrement _pending_files_count. The count therefore stays above zero for good. Consequences: the views stay suspended waiting for it (suspend_while_loading_exit is never called), and anything waiting for the count to each zero never finishes (for instance the end of a session restore).
 
The cluster_new_files auto-clustering never runs either, as it waits for the count to reach zero.

Hard to trigger in practice (the file has to be removed during its load), which is probably why it was not reported yet.
  
Expected: the callback is called for a removed file so the count is settled, and the removed file is then left alone.