InCombatWithMe

That's correct behavior. It only checks if you're being targeted by the unit and have combat state AND if you're not ignoring combat in settings.
If you want to know whether a unit has threat towards you, use the threat API (only works in Wotlk+)

4 hours ago, Matenia said:

That's correct behavior. It only checks if you're being targeted by the unit and have combat state AND if you're not ignoring combat in settings.
If you want to know whether a unit has threat towards you, use the threat API (only works in Wotlk+)

Good. But what if I want to check that the enemy player is hitting me, but is not targeting me? Only using lua events? It's a headache ...

There is no other way. If you are on their threat, you are in combat. Otherwise literally there is NO WAY. It's not in the game.