The bug

When a player in creative mode uses an empty map the item count is reduced for a short moment.

Code analysis

Based on 1.11 decompiled using MCP 9.35 rc1

The problem is that the method net.minecraft.item.ItemEmptyMap.onItemRightClick(World, EntityPlayer, EnumHand) is always reducing the item count. This causes a single empty map to be replaced with the filled map as well.
Additionally the client-side method net.minecraft.client.multiplayer.PlayerControllerMP.processRightClick(EntityPlayer, World, EnumHand) should be changed to reset the damage and the count just like the server-side net.minecraft.server.management.PlayerInteractionManager.processRightClick(EntityPlayer, World, ItemStack, EnumHand) does.