When i cleave or AOE a grp of enemyes and i fixate on one enemy, the others do not loose hp unless i fixate on them, meaning i have to stwitch target if i wish to know all of the enemy's hp.

Is this for the target frame or nameplates?

i noticed this too, definatly the nameplates
so what happens is, i pull a large group of mobs, and i chain lightning, which of course only hits 5 things but their nameplates healbars dont move at all, unless they die from my mastery double proccing the same target, and when i tab target the next target it finally shows the health (i did not noticed last night in raid if that also mean raid damage to an add affects health though) i PMd you a video in discord

Edit: nameplates also doesnt have the spell name of the cast you would be trying to interupt when you arnt looking at target frame