Sophos Central Email Advanced: S/MIME vs. Tag in Subject Line for encryption

Good day,
I am currently testing Sophos Central Email Advanced.
I want to allow users to do the following:
1. If S/MIME is available - to encrypt messages fully automatically via S/MIME.
2. If S/MIME is not available - to send unencrypted messages.
3. If the user absolutely wants to encrypt, whether with or without S/MIME: always encrypt the e-mail using a defined subject line tag.

I'm currently noticing that the subject line tag doesn't work as soon as S/MIME is turned on.

What do I have to change?
Many thanks for your help.