5D3 - screen flicker in custom modes

Hi all, I posted this in another forum but didn't get a whole load of response so trying here as well.

Just picked up a 5D3. All very positive except one curious issue. When I switch into the custom funtions (e.g. from AV mode to C1, C2 C3) the screen comes on and flickers a very dark grey. Usually when you switch between the standard modes (e.g. AV to TV or M), then the screen remains completely off, but for some reason it doesn't do this when switching into the custom modes.

Anyone else noticed this? Anything I should be worried about. It doesn't impact performance at all so I'm not worried about it per se - just seems a strange oddity!

Cheers