"Edit as new mail" after send fail always sets the identity to default identity?

My email provider sometimes fails to send mail, so I find the mail that didn't send in the sending box (发件箱; I don't know the name in English), use the "edit as new mail" option in the mail 3-dot menu (there's a pencil icon on the top of the screen, but it has no reaction) and tap send button again. I find that every time I do this, the mail's sender identity is reset to the default identity. I know my Thunderbird version is not latest and is beta, but I'm hesitant about updating. Is it a bug or designed behavior? Steps to reproduce: 1. Turn off your device's network. 2. Compose and send a mail with an identity that's not the default. 3. Find the mail in the sending box, and choose "edit as new mail". 4. Hit send button. 5. View mail in sending box, it now uses the default identity.