Forum user noted that emissive lighting being built with GPU Lightmass is in fact being built, just not around the mesh, but in a larger radius and with reduced strength compared to 5.1.1. They found that adjusting the base color of the material finally gives me at least some light. It's very dim, base color is auto clamped 0-1. Without base color connected, there is no bounce light from emissive light.
Tested regression in //UE5/Release-5.1 and the issue was not present. This is a regression.
Result: Light falloff from emissive material is different after bake
Expected: Light intensity and falloff from emissive material remain the same after bake
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-195177 in the post.