I’ve updated Spamsieve to latest version and when I now re-install the Apple Mail plugins Spamsieve no longer appears in rules. I’ve tried deleting / reinstalling / rebooting etc.
I can still see Messages > Spamsieve options and I can flag mail as good or spam but nothing is being caught on receipt.
The contents of Apple Mail’s Rules preferences have nothing to do with installing SpamSieve or SpamSieve’s plug-in. Missing or unsaved rules are typically caused by problems with the file permissions in Mail’s folders or by problems with iCloud. Are you able to re-create the SpamSieve rule? If so, does it persist between launches of Mail?