Mission Planner 1.3.56 - Waypoint altitude checkbox (relative, absolute, terrain) disappears

I wonder why randomly the checkbox to define the waypoint altitude type disappers while connecting to pixhawk. Sometimes the checkbox is visible and the type can be chosen, sometimes it disappears:

Some help would be much appreciated!

Mission Planner 1.3.56 (latest stable)
AC 3.4.4
Wint 10 Tablet (Trekstor Surftab)

it is removed because the firmware you are using does not support it. please upgrade.