Mail Sending Bug in IOS 14

I have three Mail Accounts, one of which is a POP account with three aliases. Way back I set up a Default sending account and a Default sending Account for one of the three POP account’s aliases. All has been working well until I updated to IOS 14 a week ago. Since then, when sending an email, I select the account or alias from which it is to be sent (if not the default) as usual but find that although all looks OK when sending, the recipient reports that the email has come from another of my accounts. So for example if I select account 1 to send from, it actually sends from account 3. Or 3 becomes 2 in an apparently random way. This also happens if I send mail from my iPhone SE whereas it was OK before I installed IOS 14. However, thankfully, Mail on my old iPad Mini 4 (on IOS 12.4.8) still functions normally. What’s going on?