macOS: Running action on multiple drafts – bug or feature?

I would suggest you configure a “File” action (or Dropbox, or similar action step) to write the files the way you want. There’s no need to alter the content of the draft, each of those steps to write to files has template that control the file naming and content of the file. You can insert drafts tags in those templates. Once you have an action that writes out the file with the addition content you desire, it will work fine with the multiple selection > Run Action method.