Incoming message matches multiple accounts, mail chooses wrong reply-from

I have 2 email accounts for my daughters, daughter A has a@gmail.com and daughter B has b@gmail.com
They are both set up with their own account on Apple mail.
Often, mails arriving are addressed to them both. They arrive fine, each receiving a copy in their inbox.

If daughter A reads it in her inbox and hits reply, Mail chooses the correct reply-from address (a@gmail.com)

However, if daughter B reads it in her inbox and hits reply, Mail chooses the incorrect reply-from address (a@gmail.com)

Sometimes it is vice-versa - the choice is determined by the first listed recipient in the original mail.

Anyone know how to fix this ? - aside from having to pay attention to the reply-from dropdown every time.

Thanks
Paul