**JUNK** in subject line of non-junk routed messages

It may actually be your mail server that is adding *JUNK* to your subject line and not Mail. For example, my school's server adds *SPAM* to mail it thinks is spam, and it gets delivered to my inbox like that. It's then up to me to decide what to do with it. If this is the case with you too, I'm not sure if there's anything simple you can do about it on your end. If the messages being marked with *JUNK* are actually junk, I suppose you could just add a rule that sends those messages to your junk folder; if they're not junk, it may be something you have to just deal with.

Of course this may not be what you are experiencing at all 🙂

This sounds like server-side spam filtering. Servers will tag suspected junk email with flags like this so email clients can easily filter it out. I'd contact your email provider to see if there's been a change in the server's configurations. I'm assuming this is only a subject-line modification to the emails, and not something that's marking the emails in other ways.