2-axis gimbal stabilization issues

I have 2 axis servo gimbal (pan and pitch) on my rc plane. Pan servo is 270 degrees, pitch servo is standard.

In neutral position (pan=0, pitch=0) the pitch servo works as expected compensating for plane pitch angle. If I pan the mount 90 degrees (the camera is pointing left or right relative to the plane) I expect the pitch servo to compensate for plane roll. But it keeps reacting to pitch. Is this expected? Or does the 4.3 implementation of servo gimbal support for 3D stabilization of camera gimbal taking into account mount pan angle? Here are my (related to mount) parameters:

MNT1_DEFLT_MODE,3
MNT1_LEAD_PTCH,0
MNT1_LEAD_RLL,0
MNT1_NEUTRAL_X,0
MNT1_NEUTRAL_Y,-90
MNT1_NEUTRAL_Z,0
MNT1_PITCH_MAX,0
MNT1_PITCH_MIN,-90
MNT1_RC_RATE,20
MNT1_RETRACT_X,0
MNT1_RETRACT_Y,0
MNT1_RETRACT_Z,0
MNT1_ROLL_MAX,0
MNT1_ROLL_MIN,0
MNT1_TYPE,1
MNT1_YAW_MAX,135
MNT1_YAW_MIN,-135
SERVO9_FUNCTION,6
SERVO10_FUNCTION,7
RC9_OPTION,214
RC10_OPTION,213
RC11_OPTION,163

Thanks,

Anton