Numbers will not save to iCloud-backed directory
When I try to save my Numbers spreadsheet to an iCloud-backed directory, like Documents, Numbers returns immediately and lets me continue working. However, the document name in the title does not change to the save file name, and at some point a Beachball will occur. Numbers will not recover (at least if left for a few hours) and must be Force Quit. I have also had the entire system become non-responsive after this happens and is left for a while.
I've turned off the Numbers syncing in the iCloud preferences, turned off iCloud entirely, then later turned it back on, and reboot several times. iCloud has 1.5TB free, according to the Apple ID settings dialog.
Has anyone had this issue or, especially, found a solution?