Heaven on High conseal

[Fawkes]

The player went directly infront of it and it aggrod in that video. Serverlags/ticks makes it look like the player is only behind the mob shortly before, but it passed its center for a brief moment. Also when the player moved earlier close infront of him, the mob turned. When a mob turns he won't aggro, for some reason, until it turned fully. I'm pretty sure it would aggro if you would stand at his ring at the arrow that shows its front.

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

[KageTokage]

There's five types of aggro as far as I've noticed, which are:

Sight: Creature only attacks if you're standing in front of it.

Proximity: Creature attacks if you get too close regardless of what direction.

Sound: Creature only attacks if you're running.

Blood: Creature gains a large proximity aggro radius if a target is at low HP, but is otherwise passive or contact aggro only.

Magic: Using any skill classified as a spell will cause the creature attack if you're too close to it, but is otherwise either passive or contact aggro only..

[Neri]

Did you use it when you already appeared on the aggro table?

[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.