MAX autobrake is only selectable on the ground. Because it’s only used for rejected takeoff.
In flight. MAX autobrake is always disabled. That’s how A320neo actually behaves in real life. So MSFS has always been wrong to make MAX autobrake available at any time. It looks like this is actually fixed now.
So the rule is. For takeoff, use Max autobrake.
For approach and landing, use either low or medium autobrake depending on the runway length.