Hit Detection from Front
 

Mobs have extremely variable hit detection when their target is in front of them. This is most obvious when being chased. If you run in a straight line, mobs may bash you from as much as 15 yards away. If you run at an angle, or zig zag, this almost never happens.

Presumably this is being caused by server lag, letting the mob move further along its intended path in a short period before correcting itself. This would explain why it works when traveling forward, but not when the mob is turning.

I've seen this on the bard forum, but didn't find any posts related to hit detection here. I'm not sure how well known this issue is, but it's been a problem for a long, long time.

That's how it always worked in classic - strafe running was guaranteed to let you avoid hits from almost any mob because you're moving at roughly 1.414 times the speed you'd normally be moving (since you're moving one unit forward and to the side in the amount of time you'd be moving in only one of those directions, and well... if I have to explain the pythagorean theorem that would just make me sad). A lot of people including me remember not being able to run away from mobs in a straight line. I remember I always had to run and jump.

Resolving due to age