I went mountain flying today and discovered a nasty bug.
As soon as the APM goes over 1300m ASL it immediately thinks it's altitude is -1000m.
This results in it immediately trying to ascend another 1000m.
It is also reported in the mission planner as a -1000m.
As soon as you descend below 1300m it returns to the correct reading.
I was able to replicate it twice.
Is this a hardware fault in my particular setup or is it a bug.
The only way I could get my drone back was to put it in guided mode and tell it to descend to -800m. As soon as it physically dropped below 1300m ASL then the readings corrected and I could reissue an instruction for the correct altitude.