The enmity/threat bar not appearing, despite attacking the target, is an issue that has existed since (at least) The Hunt. It pretty much happens when there are too many around the area attacking the same target. The amount of threat or damage you do is irrelevant due to this. That being said, if you are late to the attack of a V and Star rank, just like an S rank Hunt target, you may come across the problem of no threat generation... meaning your actions/damage is registering, but the game is not applying it towards your own credit. The proof of this is the fact that you're not even in combat, despite attacking. I can mount and fly around immediately after attacking a mob that is not registering the action I took (even though the damage has been done). As a healer, this means you can heal with OOC MP regeneration rates. You just won't get any credit for it.

Essentially, you're more of a detriment to your group trying to get credit for someone elses claim by attacking and speeding up the kill simply because the game does not recognize you as being in combat, and the damage you do is registered as an unknown to the encounter lol. Similar to traps doing damage to mobs in places like Qarn HM.