Auto mode does not start or starts after a few minutes

I report here a behaviour of the antenna tracking firmware which does not seem right, or hidden rules apply that are not documented.
The antenna tracker is configured in proxy mode with a pixhawk onboard the tracker that relays the mavlink info from both the copter & tracker to mission planner, via radio. That part works fine, vehicles are displayed right on screen in MP.
What does not seem to work right is the auto mode. When it is turned on (via the action tab in MP), nothing happen at all, or nothing will happen during a few minutes.
Then, sometimes (but no causes identified), only the yaw starts to track, the pitch does not track. The pitch and yaw tracking value are checked on the status tab in mission planner (the RC1 out and RC2 out values).

I noticed on the status tab the alt err has an infinite value. Is that normal ? Would not that be the cause of the pitch not tracking ? see picture.