I think that the name of the pseudo-field for the filename is _FILENAME (with an underscore). You now probably have a field called FILENAME in your tags.
The goal was and still is to have a tag with the field name of FILENAME. As it turns out, you are correct about the need to add the underscore in front of FILENAME to create a proper action item. Doing so makes it perform correctly.
I thought about this possibility but was mislead by the fact that in the list of selections for the Tag --> Filename configuration, the underscore is not shown. Had I been more alert, I would have noticed the underscore when creating the action as part of the compound Action Item we use after all of our other tagging has been completed.