I am having the exact same issue. Here are some screenshots comparing the lighting in 5.3 VS 5.5.
Scene in 5.3:
Scene in 5.5:
Not only is the lighting significantly darker, but decals don’t render properly and roughness maps stand out way more than they used to.
The settings are identical, I have tested this on two different computers swapping between DX11, DX12 and Vulkan. Did anyone find a solution to this?