MCH sync is based on the the opensource syncthing where similar problem occurred.
Temporary files are removed after 24h. However leftover syncthing.*.tmp files are a sign of incomplete or aborted sync progress, so you should probably investigate that (possible question: are the non-temporary files really equivalent to the source and are the temp files also equivalent or different?).
Most users don’t bother to check and just delete them after a while.
