When an email having "Confidential" word anywhere in subject line is replied or forwarded, "*Confidential:" prefix is automatically added at the beginning of subject even when user didn't select “Mark Subject Confidential” in “Delivery Options”.
This is a known issue and below link mentions working as designed.
However, customer wanted to avoid auto addition of "*Confidential:" prefix without user intervention.
Please consider if the behavior can be changed for both English and Japanese templates.
Instead of auto addition of "*Confidential:", the sender should decide whether the subject has to be confidential or not.