Maybe you’ve enabled Lumen, which will replace baked lighting? Lumen only works with Moveable or Stationary light source, so if your lights are set to Static they will be ignored.

For types of productions there’s absolutely no need to enabled Lit for the Billboard. Since it’s a flat surface it won’t react realistically to light, it’s much harder to control and generally just doesn’t look very good. It’s much better to light your subject with physical lights to match the virtual environment. This is just the nature of compositing 2D video in a 3D world and until the technology for real-time AI depth estimation catches up in a few years there’s nothing Aximmetry can do about it. 

First of all, thank you for the response.  Lumen was on before the update too. Everything looks darker (with no changes from my side), although I like the general feel of the image, I am just trying to understand what is going on.

Hi,

It could be that some of the lighting actors or effects were changed by Unreal in 5.1 and they have different strengths now.
Also, from the shadow, it looks like the Billboard is lit from behind/side by virtual lights. You could try if it is better when the billboard is actually facing the light's direction.

Warmest regards,