Could you provide me with a screenshot and/or asset to test so I can get this issue to appear on my end?
Possibly some steps to follow so I can see the artifact?
Cubemaps should not need mimaps as they are used for the ambient lighting for the reflection environment. You can get an idea of the default cubemap settings when using a SceneCapture Cube and creating a static texture from the cube render target.
Scene Capture Cube - Static Texture
Let me know if you have further questions or need additional assistance.
Cheers,