There is an intermittent problem with NPEs giving the message "You are no longer eligible to see this enemy." when you try to target them. I've currently got this with a Dark Ring that'd I'd already hit. At level 291 I'm well inside the eligibility criteria of 100-600. At least two others in my legion have experienced the same problem.

Is this a common problem? I've only experienced it in the last week or so.

That shows when the NPC expires (wasn't killed) and the player who alerted it hasn't logged on yet to refresh there battle tab.