Accented characters (UTF-8) in personal messages are incorrectly rendered in email forwards

As mentioned in this discussion, personal messages containing international accented characters (with diacritics, UTF-8 encoded) do NOT carry over correctly to the forwarded email copies of these messages.

Moodle's internal messages interface displays such messages correctly; however, forwarded email copies look corrupted (unencoded?).

I have tested these languages: Polish and French. I have added screenshots of the test examples tested.

I have tested directly Moodle versions: 3.9.8 and 3.11.1 (the MoodleCloud site I administer).

The bug does not affect Moodle 3.6.10.

Interestingly, the problems appears to ONLY concern the typed contents of personal messages - NOT notifications about Insights or forwards from subscribed forums.  Also the email test message sends correctly. So this file outputs what it should /admin/testoutgoingmailconf.php  , and the Polish language string "testoutgoingmailconf_message" is rendered correctly in the forwarded email of the test message.