Description:
If you perform the following steps while facing hostile npcs (i.e. attacking them face to face) they will eventually go through you and start hitting from behind. This is especially annoying while tanking.
STEPS TO REPRODUCE:
while facing attacking NPC, back up and stop
repeat step 1
This kind of NPC movement was not present in the 2019 version nor the original 2004 version. Something has broken in the NPC movement code (either distance check intervals are too long or a hitbox issue or maybe acceptable tolerance to use an attack skill has been reduced) that was not present in the original or 2019 versions for the game.
EDIT: Easily reproducible with a melee-attacking NPC, though it may also apply to NPCs using ranged attacks if such NPCs use default melee attacks when minimum range is not satisfied.
EDIT: Looking at these reports it looks like this is an extremely old bug: