In 4.27 you had to jump through hoops to get emissive to work on foliage.
This is likely the same in 5.1
If I remeber correctly you had to enable and use Light Propagation Volumes to get the foliage to actually do what static meshes otherwise just do.
What it comes down to is instancing.
Foliage is instanced.
Runtime local light can only be computed at an acceptable rate when using volumes.
For that, you need distance fields, and a bunch of other settings…
Try to follow a guide for ue4 on LPV. See if that gets 5.1 to work like you need it to.
Other than that, you are probably complaining about eye adaptation doing its job of closing/dilating the camera aperture based on how much light filters in…