That never worked, even in the old days. It was always safe to just set it up in the loop, but in many cases the loop was only one mail message, so it didn’t really matter.
Currently, I cannot get it to work at all. Regardless of how I set up the mail rule, I cannot get any AppleScript to fire and subsequently work on an actual message. The only workaround I have been able to succeed with is that incoming messages that need some AppleScript massaging are put into a specific mailbox. I developed a launchd agent that checks every 15 minutes, and if there are messages in that mailbox, the AppleScript fires and works on them.
I cannot stand it, but it does work.
I have still not received any significant feedback from Apple pertaining to the bug report that I filed on this matter.