My iCloud email addresses gone from MacMail

I have five different email addresses affiliated with my iCloud account and use each to send/receive on Mail.

This morning I went to send an email from Mail (on my Mac) and was advised that there are no email addresses affiliated with the account, and was directed to change the Hide my Email settings. Dug in a bit deeper and all of my email addresses are gone from the Mail setup. Mail is still being delivered to those email addresses, but I'm unable to add them to the list of accounts that I can send FROM.

I was able to add my primary email back into Accounts in Mail, but am unable to add the other email addresses. When digging to try to find out why, I keep getting referred to iCloud +. Don't know if this is related - the introduction of iCloud + and my email addressing disappearing.

Help with this please?