Not able to set EKF2_EV_POS_Z offset parameter

Hi all, sorry if this is the wrong place to place this question.

I have a problem setting the position of the vision sensor (MoCap markers) with respect to the body frame.

When I set values the offset values in the EKF2, then only changing EKF2_EV_POS_X and EKF2_EV_POS_Y have the desired effect on the position reading, but when changing the vertical offset value EKF2_EV_POS_Z then the z reading in the position is not at all affected.
How to correctly input the vertical offset value?

Thank you.