Well, that's not possible. Would you please describe in more detail what happens?

It's never been an issue and I agree it should not be possible. I use a number of email addresses depending on whether business, personal or other. I fill in the correct from address if it is wrong. After I send it the email shows in the wrong send folder and Thunderbird has changed the from to a different email. A search shows this can be fixed by setting the default. That no longer works, I can't change it. I have started using Gmail, which I hate but it works as expected. This has worked correctly for many years but this problem has cropped up due to a recent patch. It has caused some explaining since people who should not have the company email now have it. Embarrassing.

So Thunderbird changes a customized from address, and you and your recipients see a from address that is different from what you typed. Is that right? That could give a person the impression that Thunderbird was changing data after sending a message.

I'm not familiar with the use of customized from addresses. I use aliases and identitities, which seems to me to be a better for sending from a different address on the same account. Have you considered using them? You would not have to keep typing the same from addresses.

I have not been able to learn much about customized from addresses. Let's hope that other people know more.

What version of Thunderbird are you using?

If a non-gmail account is set to send on a gmail smtp, the From of the non-gmail account is automatically switched to the gmail address, so the recipient sees the message as having been sent from the gmail account. The remedy is to set the non-gmail account to send on the smtp with non-gmail settings, i.e. the smtp User Name is the non-gmail address.

https://support.mozilla.org/en-US/questions/1350593

https://support.mozilla.org/en-US/questions/1438310