It happens with all mages (or any class, I didn't test that yet) that use a quarterstaff (I don't know if this is the right English word for it).
It is very complicated to describe. There are 3, I think, animations for a Mage using a staff. He can hit from up to down, from one side to another or he can "stab".
Now, the Bug appears only, when the mage "stabs" AND ONLY when you see his avatar from behind. So the bug doesn't appears, when you see the mage's avatar from the front, ONLY from behind.
here some screenshots:
1st with normal attack animation
and then second, with the missing animation when he stabs:
