AE and Ps manuals have the following explanation for Linear Light in the Blending Mode section:
"If the underlying color is lighter than 50% gray, the layer is lightened because the brightness is increased. If the
underlying color is darker than 50% gray, the layer is darkened because the brightness is decreased."
Well.. this is not the case at all.
1 Because you could have the underlying layer with 60%,70%, 80% etc brightness, and have a darker result, with lower brightness. As long as the
added brightness of the two layers do not go above 100%, than the manual explanation is wrong.
Ex. Above layer 20% brightness / Underlying layer 70% brightness = result 10% brightness ( so..is darker)
2 You could have the underlying layer with 20%, 30% etc brightness and still get a lighter result! As long as the
added brightness of the two layers do go above 100%, again the manual is wrong.
Ex. Above layer 80% brightness / Underlying layer 30% brightness = result 90% brightness ( so..is lighter).
How are you figuring the brightness? AE doesn't have a brightness property, it has rgb values. It may also have hls values, but I never use them.