Mac Mail Uses 'Find My Email' When I Didn't Ask For It

I'm experiencing a weird bug where Mac Mail (Ventura 13.6.6, Intel) is changing the 'from' address in my emails to 'Hide My Email' addresses, without warning.

If I notice and change it back, when I send I'll get a couple of error messages, the second of which asks me to choose which server to send from. I choose the iCloud server, and it now sends, and I can see in the 'sent' folder that it is sent from my normal address... but the recipient still reports getting it from the 'Find My Email' address.

This happens several times a day and, with all the error messages etc, feels like something has got corrupted somewhere. I can't see anything wrong in the settings. On my iPhone and iPad, there are no problems.

The obvious thing would be to remove iCloud email from my computer and the reinstate it fresh, but I can't find a way to do this without removing iCloud completely, which I really don't want to do! The Mail app's settings will allow me to temporarily turn off iCloud mail, but won't allow me to remove it as it says other apps are using it. Under iCloud in System Settings there is the option to turn off mail, but it seems to be temporary again (it reinstates instantly so can't be removing anything).

What else can I try? Are there preferences I can safely delete?