Reference: Windows desktop app Sync replaces any literal "%20" in names with literal space (October 2022).
Recap: the Sync feature replaces all occurrences in source filenames of a percentage sign followed by a number with a unicode character in the destination filename. For example, “Very%20Important%21File%22Don’t%23Rename” becomes “Very Important!File1Don’t#Rename”.
I’ve also noticed that if we use one-way instead of two-way sync to avoid it also renaming the source files on the next interval, it will repeatedly copy such files as if they were new.
I’ve been using the app’s virtual drive with robocopy as a workaround, but that runs into the dreaded “1m timeout restriction” for small files - the drive transfer mechanism can copy any number of multi-megabyte files in a row without blinking (which, kudos) but give it any multi-kilobyte files via robocopy and the app may or may not fail to transfer them - and, unfortunately, if it does fail it will always fail the same way each time so I can’t just run the robocopy command again to catch only the ones it missed the first time.