Syncthing is creating thousands of tmp files endlessly

I have a special use-case that syncs files created by “Steam ROM Manager” in folder …/steam/userdata/29478602/config/grid. This folder contains 10000+ small image files. For some reason some devices (not all!) create thousand of ~syncthing…tmp files. CPU is getting hot. No idea why. How to debug?

Check the logs. Perhaps the files are appearing/disappearing or changing too quickly, so syncthing starts syncing, and the files disappear, in which case syncthing leaves the tmp files around for 24hrs just in case the file comes back.