Outlook 2016 - chinese characters appearing in messages when replying/forwarding

Hello,

I am investigating an issue with Outlook 2016 and seemingly random appearances of chinese characters in email messages when replying or forwarding. I have a Exchange 365 hybrid environment and O365 pro plus office installed on workstations.

I have not been able to pinpoint a common factor why or when this happens. It is possible to reproduce the issue by sending a plain text message to a O365 group and then reply/forward the same message > header information is transformed into "chinese" (I have no idea what they actually are) characters. This does not happen with all groups or senders which is why I am looking into Outlook at the moment.

I`ve tried to change Outlook settings: use UTF8 with mailto protocol, change the encoding settings to different options etc. without success. 

I`d like to hear how would you start to troubleshoot this issue?

BR,

Sam