What title says.
When I play a Custom game on Summoner's Rift or The Crystal Scar the map is all dark. On Howling Abyss and Twisted Treeline looks normal.
Haven't tried yet on PVP and Coop, so no idea if that happens there too.
1. Create custom game.
2. Select Summoner's Rift or The Crystal Scar game mode.
3. Enter the game.
Here are some screenshots (gotta admit I love this bug, it looks cool xD).