If I put my preferences in that state I get nagged to enable Siri every morning. I'm on 10.15.5 (19F101).

I guess I'm in the wrong leg of the A/B test. Fing fantastic.

This sounds like you may have a corrupt preference file somewhere - perhaps deleting ~/Library/Preferences/co. On a normal install, you'll only be asked to enable on major OS releases. Since you're also on an old version of Catalina it'd be a good idea to install the security updates, too.

You could try to delete all of the preference files:

    find ~/Library/Preferences/ -iname com.apple.siri\* -delete