PGP doesn't work with S/MIME?

We are using PGP encryption in our company, which is working fine. I imported the public keys into emClient, which also worked fine. However, I noticed that when my colleagues sent me mails where they had also encrypted the subject line of the e-mail, I could decrypt it, but no text would show. As soon as they changed their settings (deactivating the subject line encryption) it worked.
Just wanted to know if this is a common thing or not. It seems to be connected to S/MIME if I’m not mistaken. If I am mistaken, feel free to enlighten me :slight_smile: