https://www.curseforge.com/wow/addons/threat-plates-classic/issues/42

Starting from a default profile I want to enable headline view for friendly players and "enemy players I cannot attack" I expect this to show the Headline nameplates for all players (horde and alliance) while in shattrath, and then revert to default functionality in the open world. The reality is when I go to "Headline View" > Show By Unit Type > Friendly Units > Players, ENEMY units (opposite faction) are forced to headline view, while nothing happens to friendly units (my faction). Similarly, Headline View > Force View By Status > On Enemy Units You Cannot Attack, has no effect whatsoever.

There is some weird interaction between enabling friendly/enemy nameplates and what is actually shown. I noticed that when TBC Classic was going live. I never really investigated this, but it could be related to that. I'll test what you wrote above and see if it's fixable. Might take some time though ...