This is also happening for replies I receive that are sent via webmail.

Thank you for reporting this, it has been added to our bugs list.

I wonder if this is part of the issue a client of ours is seeing with "Failed to update headers" responses in Outlook.

Alex, are you still receiving messages where the UTF8 identifier characters are showing in the headers for forwarded messages?  I'm no longer able to replicate the issue against the latest public release and believe it was fixed by a previous fix. If this is still happening, please send me a personal message.

Chris, are you still getting the "Failed to update headers" in the IMAP response between SM and Outlook?

I've requested more information from the client Robert. Josh I believe has the screenshots etc. in his DMs from me.