I have the impression that smart rules can only be triggered by “user-initiated” actions.
I’m aware that one of my smart rules, which has the On Import trigger, applies when I drag and drop files into the inbox. It does not apply, however, when files are imported via the AppleScript command import, unless the script explicitly calls the smart rule through a perform smart rule command. I think it’s because AppleScript import is not considered a user-initiated action.
In the OP’s case …
… the new tag is not “moved” into the database. Rather, it is created automatically when DT recognizes that a tag of the same name does not exist yet in the destination database. Since the creation is automatic (not user-initiated), the smart rule’s On Creation trigger does not apply.
Please do correct me if I’m wrong.