I am having a similar issue. Some messages marked "QUARANTINE" do not end up in the spam folder or anywhere else. If I turn off the spam folder check box, some message still go in spam folders and some go in the inbox.

If I look in the logs, messages that are "quarantined" are not being placed in the "Spam" folder. I tried turning off the spam folder so only subject re-writing is taking place, and those messages don't appear anywhere either. Version:2.1-20.6 OS Ubuntu 22.04.3 LTS Product Plesk ...