robot sinks under ground if I change origin x/y in height map

edit

In a height map if I change the origin position from <pos>0 0 -2</pos> to some<pos>x y -1</pos> robot sinks inside the terrain(height map). This doesn't occur if origin x,y is at the center. Is it some kind of bug or Am I doing something wrong? psuedo code of sdf file:

 <model name='heightmap2'>
   <static>true</static>
  <link name='link'>
      <collision name='collision'>
        <geometry>
            <heightmap>
            <uri>file://media/slope_15deg.png</uri>
            <size>25 25 5</size>
            <pos>0 0 -2</pos> 
            </heightmap>
        </geometry>
      </collision>
      <visual name='visual_abcedf'>
        <geometry>
            <heightmap>
            <texture>
                <diffuse>file://textures/img.png</diffuse>
                <normal>file://textures/flat_normal.png</normal>
                <size>1</size>
            </texture>
            <blend>
                <min_height>2</min_height>
                <fade_dist>5</fade_dist>
            </blend>
            <blend>
                <min_height>4</min_height>
                <fade_dist>5</fade_dist>
            </blend>  
            <uri>file://media/slope_15deg.png</uri>
            <size>25 25 5</size>
            <pos>0 0 -2</pos> 
            </heightmap>
        </geometry>
      </visual>
  </link>
</model>