Thunderbird sometimes renders decrypted content in its encrypted form when it is quoted in replies

Question

I'm using Thunderbird with built-in OpenPGP encryption. It works but I have a problem when I receive mail from specific accounts.

Usually, when I receive an encrypted message, it's decrypted on the fly when I click on it (I don't see the encrypted content with the "Decrypt" button); when I reply to the message, the quoted message text is displayed unencrypted, and the entire message is encrypted once I send it. And that's what I want.

When I receive messages from certain senders, it's also decrypted on the fly when I click on it, but when I reply to the message, the quoted message text is displayed in its encrypted form, as seen in the following image:

What's wrong with the message? How can I reply to it and quote the unencrypted message text as is typically the case?

Answer 1

I've found the root of the problem. By analysing mail envelopes I've noticed that Thunderbird was sending the encrypted mail in utf-8

...
Charset: utf-8
...

That was different from the charset used by the mail that I've received (as you can see in the question image is iso-8859-1. I've asked the sender to try to change the charset in utf-8 in the options of his mail client, and everything started to work again.