What can cause ATT.Pitch be very much off ? (then be ok)

I discovered that ATT.Pitch is temporarily unrealistic, and EKF1.Pitch is, still - This happened a few times in FBW* modes, and I’ve never seen anything similar in Auto (maybe pure luck ?)

This is a new plane with a new pixhawk, It’s really about this thread, viewtopic.php?f=107&t=12012