We seem to have a bit of strange behavior. We were testing indoor flight with an IRIS quadcopter with optical flow. We noticed the HUD shows drift even when the vehicle was at a stand still. After repeated calibration attempts with and without optical flow failed to fix this, we flashed the firmware. After flashing, the horizon was stable but the flight itself was not. In position hold ( with optical flow ) there was a noticeable drift. Finally, we upgraded to the V1.6.0 RC3, where we also get a stable level horizon at stand still but the manual flight is not stable. I have attached the log from the last flight. Could someone please tell us what could be the problem. Thank you.
Our IRIS quadcopter with V1.6.0 RC3 and QGC v3.1.3