A torch. This torch has the following checked:

dynamic
spot light ( this may have some effect )
spot shadow ( this may have no effect )

The torch is held by your scumbag enemy. You violently end their life and they drop the torch. The torch now has half dynamic lighting. It is lit and it shines lights on walls. But the floor is unaffected.

Here, you save and reload.

Upon reloading you see that that same torch now has full dynamic lighting. It lights the floors and walls and objects, and you can pick it up with Z and the light will follow.

What has happened in the save and reload that enabled this? And why does the torch switch from full dynamic lighting in their hand to something that is broken and on the floor until a save and reload intervenes?

It's a common engine bug in the lighting system. Nothing to be done about it other than what you did.