Gotta add some info from my perspective...
My Joplin instance is portable and running on an USB device connected to a hub and also connected to a monitor hub... I know, not ideal, but hear me out... 
Once for awhile, for example when entering into sleep mode, the USB device disconnects "accidently" while the instance is still running... that's when I also encounter that situation where the sync process doesn't finish and can't be cancelled either.
That's all I would like to provide for more insight into that sync problem. I'm not a pro coder... yet. Just an observer and still happily using the Joplin app for different reasons and purposes.
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