Michael - for some reason SpamSieve is not appearing on the list of installed plugins for Mail. I tried to uninstall the plugin for Apple Mail and re-installed it but SpamSieve is not appearing.
I’m running Mail 16.0 (3696.120.41.1.1) on Mac mini (M1, 2020) with macOS 12.5 and SpamSieve 2.9.50b1