How to make a cube map to reflection probe?

if i set a reflection probe as “custom” type and put a cube map into it, everything rendered with this reflection probe will looks so smooth and reflective, even if i turn on the midmap. What did i do wrong? now i am using a dumb way to do that. I generate a mesh, add the cube map on it with special shader. And call RenderProbe(). It’s working but it needs extra layer, and probably some rendering that is not neccesary. is there any better way?

If you use the import settings to import a images as a cubemap then make sure to check the “Glossy Reflection” option in the importer, only then unity will generate the necessary mip maps that will be used on not so smooth surfaces.