I have done 3 more tests
Test 1. LSZH (MK Studios v1.0.5) parking position P36 passenger PW version in house livery same result the plane moves forward by itself I am not even in the plane.
Test 2. LSZH (MK Studios v1.0.5) parking position P36 passenger PW version in house livery without GSX same result.
Test 3. LSZH (MK Studios v1.0.5)other parking position E55 no problems.
maybe the terrain at parking position P38 is not completely flat and therefore the airplane rolls forward, braking does not help