For diagnosis, the output of the /rest/debug/file for one of the files shown as out-of-sync would help. It takes the same arguments as this endpoint, however you’ll have to enable debugging first (web UI: actions > advanced > gui): GET /rest/db/file — Syncthing v1 documentation
Here’s an example curl command for linux: REST API — Syncthing v1 documentation
And an explanation for powershell on windows: [ How To ] call the REST API from Windows PowerShell
After that I’d try starting Syncthing with the -reset-deltas command line flag on primary.