This happens in doom2.wad (standard mode):

This happens in the same place (dark mode):
Spoiler:You can't see almost any difference (or any difference at all), which is weird.
Dynamo made these pics:
This is a sector with brightness 0 in an OLD version of gzdoom ->
Spoiler:This is a sector with brightness 0 in a modern version ->
Spoiler:The weapon got darker but very little, which is weird because that is a place with brightness 0.
The only way to darken enough the weapon in gzdoom 2.2.0 is using sector light mode in "software".
Brightness 0 in gzdoom 2.2.0 64 bits (dark mode, but the same happens with any other mode that is not software):
Spoiler:Brightness 0 in gzdoom 2.2.0 64 bits (software mode):
Spoiler:So, why does this happen? Is this intended or is it a bug? The prettier situation for brightness 0 would be the last one, but you need to choose always software mode for that. Thanks and sorry for the storm of pics.