Looks like it was Magento, using Zend Framework 2/Laminas:
https://github.com/magento/magento2/issues/29258
https://github.com/laminas/laminas-mail/issues/2
It looks like the problem has been fixed. We could teach mhfixmsg to
fix this, though I'm inclined not to do that unless this turns out to be
a more widespread problem.
It's up to you, David, but according to the first bug report it broke things
for Thunderbird and Exchange/Outlook, so I think this is something that
should be fixed rather quickly.
--Ken