Placing a rect light in between two static meshes causes the surface to be extremely bright. This will happen even though the lights settings are default. It will not happen if it is lighting just a single surface.
Found in 4.20 Preview 1 CL# 4112139 and 4.21 CL# 4115885
Alternatively you can use the project attached using the steps below
The surfaces being lit by the Rect Light will be incredibly bright even though the settings of the light are default
The surfaces will be evenly lit
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-60104 in the post.