Heaven on High conseal

[Commander_Justitia]

The player walks back and forth in front of it from 0:16 to 0:30 and it does not aggro.

Lmao, no, the part you describe, there is a distance between the mob and player. It is not "directly" infront of it at his hitbox. He actually aggrod the mod at 0:38 but servertick makes it look like 0:40.
The mob should have aggrod at 0:28/29 too, but that is the moment the mob turned, mobs are blind the moment they turn (even sight aggro mobs) until they completed the turn, the positional updates have a delay/lag so it looks funky in that video. If you would do the same movement without the mob turning it would aggro too.

It's probably that both our facts are kinda right, mobs in dungeons/duty will behave like I said but mobs in the open world seem to be friendlier, where you have to stand 1y infront of their center within the circle to aggro instead of just touching the front of their hitbox.