The bug

If you have two maps in one slot, and you open one of them, the filled map will be put in another inventory slot. But if you set a clock clearing all filled maps, the map which opens in another slot will be glitched out.
The glitched map got these properties:

• In creative, the map can be moved around in your inventory, but can't be cleared with commands.
• If moved around in inventory, the /clear command will trigger, giving the output that it cleared 1 item from you, but the glitched map is still there
• In survival, if you try to move around the map, it will disappear
• If you drop the map, it will instantly disappear

How to reproduce

1. Put down a command block
2. Set it to repeat, always active, and use this command:

   clear @a filled_map
   

1. Then, give yourself 2 maps and put them in the same hotbar slot
2. Activate one of the maps
3. The activated map is glitched out

Please watch this video for a demonstration.