I have a unidirectional receive sync on a machine which is always shut down cleanly yet I see a lot of files called .synthing.original-name.tmp . In my case gigabytes of them months old when the server is only up intermittently (otherwise a waste of electricity).
Please have a scan for and remove them and report back whether you have them as I do.
Yes I’m running a reasonably current version 1.24 (FreeBSD pkg) right now.