On TrueNAS 25.04+ the middleware/docker silently aborts syncthing migration operation.
See my other post from before I found this thread, or this github issue with a very hacky workaround. Deleting old database works as well, but I wanted to prevent that.
As for logs from syncthing: the migration is reported as running (ref logs here), but no new messages keep being logged, which in turn means the migration is not running at all.
If Syncthing replied on /rest/noauth/health while migration is in progress, it might mitigate this issue, but I have not tested it. The internal test command is plain: wget --quiet --spider http://127.0.0.1:8384/rest/noauth/health.
@calmh maybe this^ would help.