Hi, today I've done my 4th or 5th flight, on a big open field in  order to try dronie and circle. The thing is that when the AC is quite high (20 meters or more), the app starts to lose altitude control, showing 6, then 4, 10, 15, minus 2!!!, 20...

That is, the height control went crazy.

Someone has had the same issue? What can it be?

haven't seen this before .. have you updated your firmware and have you calibrated your compass, IMU? or has it asked you to calibrate.  Might also want to make sure the bottom sensors and camera are clean of finger prints or dust.

Firmware updated today. As a strange thing I was promted with "gimbal overload" or something like that before take off. I just restarted AC and it went away. I had to calibrate compass too. Whats IMU? Nice point to check sensors and camera, thanks for that.

Here is a video about IMU calibration for your reference.