Is this a mesmer thing, or a bug?
My best guess would be that some sort of lag would be a factor.
For example, a couple weeks ago WvW was acting a bit laggy for a bit of time. A party of my guildies and myself were raiding supply camps/dolyaks, and there was one dolyak that acted exactly how you are describing that Mesmer. Fortunately for us, my Illusions knew where the real dolyak was, and the hitbox for the dolyak was probably about 2000 range units further ahead of the graphic of the dolyak. We then proceeded to swing wildly at where my Illusions where attacking and then killed the dolyak.
That happened a total of one time.
Perhaps there was maintenance going on at the time, and that Mesmer’s hitbox and graphic were in different places.