When cubemap texture is uploaded with GL_LUMINANCE format, sampling it behaves as if the texture had single channel GL_RED format. GL_LUMINANCE should produce grey scale texture.
GL vendor: Imagination Technologies (Host: NVIDIA Corporation)
GL renderer: PVRVFrame 10.1 - None (Host : GeForce GTX 760/PCIe/SSE2) (SDK Build: 3.5@3510720)
GL version: 3.0(Host : 4.5.0 NVIDIA 355.82) (300)
GL shading language version : GLSL ES 3.0(Host: 4.50 NVIDIA)