The bug

The gamemode target selector argument can normally only be written once, as a player can only be in one game mode at a time; this behavior can also be seen when using two type= arguments, as an entity can only have one identifier. Trying to use two m=! arguments within a target selector fails however, even though this should be valid as a player can possibly be not one of the currently five game modes available.

Steps to reproduce:

• Set your game mode to Creative mode.
• Go in game and try running the following command. This should return "Test passed," as you are not in survival nor Adventure mode.

  /execute if entity @a[m=!survival,m=!adventure]
  

→ "Duplicate m selector arguments"