The pathing has been screwy since it was first changed back in august
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Pahing before it was changed in august was good, but I guess it was too easy to exploit pathing because mobs would follow paths very tight.
First change made mobs warp through walls in HS etc. Later changes made them fall through the world. Recent changes I have seen have made mobs dart through the air (if you are levitating and trying to exploit moat pathing in CoM for instance) instead of following the "real" path of the bridge.
I don't know if some of it is intentional (and the mobs falling though the world is just "collateral damage") to avoid the pathing exploits though.
As a side note....this happens with charmed pets too :/ They sometimes fall through the world too.