unwanted rotation of the markers detected with ar_track_alvar

edit

hello! I discovered something weird with ar_track_alvar but maybe I just forgot something when launching it. I put a mark on the head of a robot and another one one the ground, this is when it is working fine:

but if i make the robot turn a little bit mor to the left, a PI rotation relative to the green axe seems to be made :

I set up a static_transform_publisher between "/axis_camera" and "/map, then I set the "output_frame" argument of ar_track_alvar to "/axis_camera" and the rest of the launchfile is identical to "pr2_indiv_no_kinect.launch".

The problem is not happening with the mark on the ground if i make it turn.

If it can give a clue, here is the rotation data of the mark when my robot make a complete turn:

I am grateful for any help!