ZDoom

I see. Thanks for letting me know.

I guess I'll just use the sky flat for all concerned sectors. At least Transfer Ceiling Brightness doesn't have the same quirk.

This is a deliberate implementation, not a bug. Don't ask me why Randi deviated from Boom here and copied a trait from the Build engine.
If this is addressed it'll probably need a compatibility setting.

In my attempt to have a scenery where walls, floors and ceilings are affected by a glow effect but [i]not[/i] the actors standing within, I resorted to using the Transfer Brightness specials ([url=https://zdoom.org/wiki/Transfer_FloorLight]floor[/url], [url=https://zdoom.org/wiki/Transfer_WallLight]wall[/url] & [url=https://zdoom.org/wiki/Transfer_CeilingLight]ceiling[/url] variants).

For some reason if a sector is under the influence of Transfer Floor Brightness and its ceiling is not the sky flat any actors standing within will be affected by the control sector, but if the ceiling is a sky then the actors are not affected. (Which is what I want.)

The test map has 3 special sectors:
The left one has a sky and is under the influence of Transfer Floor Brightness. The medkit within isn't affected.
The middle one doesn't have a sky and is under the influence of Transfer Floor Brightness. The medkit within is affected.
The right one doesn't have a sky and is under the influence of Transfer Ceiling Brightness. The medkit within isn't affected.

Tested with GZDoom 4.3.3 and g4.4pre-87-g9daec9dc4