Mail not allowing me to customize sender address

New thing this morning, the app is not allowing me to put in a frequently-used mask address to either send a fresh new message or respond to a message from a site I subscribe to. When the message template opens it limits sender address options to 1) those registered with my personal Apple user account or 2) the opportunity to create a NEW one via Hide My Mail. Totally unable to drop in the existing masking address. Never before has this been a problem.

I do not see any control options under Settings/Apps/Mail.

I’ve searched for prior reports but not seeing anything. Any help appreciated.

Cheers

Al G