Dynamic created Skylight cause light leak in interior room

Make a simple room by 6 boxes, then encompass this room by a box reflection capture, change its “Reflection Source Type” to “Specified Cubemap”, and the cubemap used is almost black everywherer. After build, inside room is total black.

After hit play, my blueprint code add a skylight to scene. Then I see room is lighting by this skylight.

I have thought box reflection capture will override skylight, so add a skylight won’t light room inside.

Am I wrong?