Examples:
1) Duneripper @ Northern Elsweyr.
2) Ice wraiths.
3) Mobs that jump into sky and then drop down, similar to first boss of RoM.
Similarities: They just disappear, either jumping into sky, or burrowing into ground, with you UNABLE TO TARGET OR DAMAGE THEM for a period of time.
On bosses, this is ok because bosses are one-time things. But mobs you have to fight amass.
Please stop this non-sense in future contents.
I have not played the Greymoor PTS yet, but I fear I would see a lot of ice wraiths there.