Using Classic Outlook on Windows for personal email, calendar, and contact management
If you say that this happens to O365 Pro Plus, so this should happen to all who uses O365 Pro Plus. I think this is still broad, does this happen on 'Deferred Channel' or 'Current Channel' versions? What build versions?
I have same issue with one of our clients who gets the exact same "Chinese Character" line. Issue happens whenever someone within the organization send email using a Smartphone through any of their Office 365 groups. Members who received that email can view the emails just fine. But when one of the recipients, who SPECIFICALLY uses an Outlook client(O365 Pro Plus Current Channel), click the 'Reply' button, the issue occurs.