Players with Permission level "Custom" can change their game mode without having Operator.

While I was hosting a game with friends in Survival, one of them was able to set their game mode to creative and fly around, and break blocks in creative, but they could still die as if they were in Survival. Everyone was in survival, but their permission level was "Custom" because i removed attack players. I was the only one to have the Operator Permission Level, but he was still able to change his own game mode to creative. I have found this issue on Windows 10, Phones, and Xbox One. I believe that this is a bug, and could you please fix it? Thank you.

Edit: I found this bug on the new 1.6.0 Release as well.