Just posting here for the benefit of others who might have this problem.
My syncthing across linux and android has been stuck with “out of sync” status for pretty much years, and I’ve finally figured out what it was.
I had files with filenames on the linux side that the android side won’t accept.
Specifically pipe (|) and question mark (?).
By renaming the problematic files on the linux side to no longer have those characters in them my sync has finally got to “up to date”. 
Yeah, this is a well known problem. Android uses VFAT for its internal storage (i.e. the /sdcard partition), so all filename limitations of the FAT filesystem apply to it (e.g. see https://averstak.tripod.com/fatdox/names.htm).
I wonder if there’s a user-friendly way to notify the user in this kind of situation, either on the sending side or the receiving side. Other than looking in logs, I mean.
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