Cannot select iCloud email address as default - Mac mini 2018 Monterey 12.1

Suddenly, in Mac Mail I can no longer send email from my iCloud email address.

It was previously my default email address. Has been for years.

In a new email, it didn't even appear in the list of From email addresses.

In Mail | Preferences | Composing my iCloud email address does not appear in the "Send new messages from" drop-down list.

In Mail | Preferences | Accounts, the iCloud account was showing as online, but weirdly the address was now that of another non-iCloud account. I changed it back, but it made no difference, except that now when I create a new email, the iCloud address does appear in the drop-down list, but the default account has changed to an account I hardly every use.

I cannot set my iCloud address as my default email address - which it has been for years.

This is extremely annoying and time wasting.

How can I fix this?