Contacts email addresses don't transfer to Mail in some cases

I'm running Sequoia 15.3.1 on a M4 iMac. When I am composing an outgoing email now, in some cases email addresses don't show up in the "To" space when I click on the + at the right and click on the email address shown in Contacts. Some do and some don't. It means I have to enter the email address manually by typing it in or pasting a copy. This is a first after many years of experience with macOS.

This is also the case with my M1 MacBook Pro also running 15.3.1, so this seems to be a software problem. My iPhones and iPads work properly in that all the addresses show up as they should.

Pretty clear it is a Sequoia 15.3.1 problem.

This is not an earth-shaking big deal, but very odd. Can anyone explain what is going on and offer a fix?

Thanks in advance.