It happens somewhat fequently to at least 3 specific Guardians, one inside the Kao Makagh Shrine and two inside the Kaam Ya’Tak shrine (in front of the stone door, middle upper pic).

The odds of it happening for those are around 25% so on average^[1], provided the Scout was standing next to a wall or other obstacle. In theory, every fourth time you kill it directly next to a wall, door, pillar etc., it will drop its spear like this.

However, those aren’t the ONLY Scouts this can happen to - basically all spear-wielding Guardian Scouts in the game can drop thir weapons like this, including the ones inside of the Divine Beasts (lower right corner) and “Test of Strength” shrines (lower middle pic).

It is of course neat when it happens for the first time, but it is not “rare”, contrary to popular belief.

Why does it happen?

Guardian Spears (all types) are just a little wonky. It becomes apparent if you try to take a picture of one for the Hyrule Compendium and can’t seem to find the proper angle for the Camera Rune to register the spear. It is also noticable if you hang one on a weapon mount inside your house, where it will have a visisbly different alignment than other spears.

The Ancient Spear from the Akkala Tech Lab seems to be unaffected.