There was a bug when I turned a friend of mine to Member or Visitor, and they still had the ability to change their Default Game Mode to Survival, Creative, etc, Maybe probably because on their screen, they were still apparently Operator when I had changed them to Member. I had tried putting them to visitor and back to Operator and this bug is still happening. However, technically they are still IN Survival, due to being able to take fall damage, lava damage, etc. However, they can break blocks easily and they dont die. This world has Caves and Cliffs and Experimental Gameplay on.