Apps opening at startup, even when told not to

Hi,

I'm having an issue with apps starting up when my Mac turns on.

My Mail app will always open when I turn my computer on. I have "Reopen windows when logging back in" unticked when I shut down or restart my computer, and I've unticked "Open at login" in the Mail app options.

I've tried fully quitting Mail before restarting my Mac, but Mail will always open at login and "Open at login" will always be turned on for Mail. I'm not sure what to do.

I had a similar issue in MacOS Mojave, but the issue seemed to be fixed on its own. But its come back and is quite irritating to deal with.

Happy to provide any more information if required. Thanks!