Hey !
In 4.27.2, if you checked “Is Sky” in your unlit material, it will be rendered in Path Tracer but ignored in lighting contribution, to avoid double count the illumination of your material in a skylight scenario.
This is mentioned in the official unreal documentation here :
In 5.0, the Is Sky material is completely ignored and rendered black with Path Tracer.
Am I missing something, or does everyone have this problem?