[Script v2019-05-24; pRetryDelay reset to “5”; SpamSieve 2.9.39; macOS 10.15.4; FastScripts v2.8.1(913); only used keepboard shortcut set to Control-Delete and shows as such in FastScripts’ Script Shortcut; script is in ~/Library/Scripts/Application/Mail; On My Mac Spam folder; SpamSieve is 1st of the Mail Rules and enabled]
At some recent time, invoking Control-Delete shortcut while viewing the Spam folder properly deletes read e-mails there, but is is moving unread e-mails there to my Trash rather than deleting them.
Having the same issue but I only heard of and started with SpamSieve after I upgraded to 10.15. It took me a while to get it running properly (four accounts) with much greatly appreciated assistance from Michael) The only issue is deleting over 100 spam mails each day. It’s really tough to want everything but it never hurts to ask…LOL Hopefully a fix will come along before too long.\
You could make a smart mailbox that shows all your spam so that you can delete it all from one place. Or let it collect for a week or month before bulk deleting it.