My better half has her own mailbox on our account and receives a lot of spam email. They are all marked with [SPAM] in the subject line but only some of them actually go into the Spam folder and will be delivered into the inbox.
That all depends on where / who is doing the spam profiling. If it is the Plusnet spam filters, you need to review the settings. Check aggressiveness and obvious spam settings. If the email is just suspected spam it could be given the spam tag but not moved.
If it is a third party (eg your email client or AV software) then you need to refer to their user guides.