Well I'm glad I could help, even though indirectly.
It's true that the Trigger Event treats "Untagged" as a tag, but that doesn't mean that it should have fired the event when you specify another tag.
Looking into the action it checks it like: "If the other GameObject has the exact same tag as the one specified by the user, fire event."
That should normally exclude "Untagged" ...weird.
If you consider your problem to be solved, you might wanna add "[SOLVED]" to the title for others to see if your inquiry is still relevant.
Have a great time and keep on creating,
Deek