Looks like Transfer brightness effect is applied incorrectly when control sector has sloped ceiling and it's ceiling height is higher than actual sector's ceiling height.
ZDoom 2.8pre-1312:
GZDoom g2.1.pre-752:
			
			
						[pre-752] Transfer Brightness Level bug
Moderator: Graf Zahl
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[pre-752] Transfer Brightness Level bug
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				Graf Zahl
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Re: [pre-752] Transfer Brightness Level bug
Cr@p. That's not fixable in the hardware renderer. It has a strict rule that 3D floors may not intersect with each other and with the actual ceiling or floor. And that's precisely what happens here.