Not sure if this has been reported yet, but the shiny code will only work in the grand underground if you hold L prior to entering the wild Pokémon rooms. If you hold L while in the room it will NOT make shiny Pokémon appear. Holding L BEFORE you enter the room will make ALL Pokémon shiny that are in the room.
After entering the room while holding L, holding the L button is not required to encounter shinies.
After entering the room while holding L, holding the L button is not required to encounter shinies.