Maybe https://forum.syncthing.net/t/syncthing-v1-15-0-security-release-v1-15-1/16621/2?
If you happen to have been using the --config flag, then upgrading to v1.15.0 would make it fail to start (and thus also fail to upgrade to the fixed v1.15.1). Although, you are saying that the processes were still running, which should not be the case with this issue.