So I think my suspicion of an ESC desync is not valid. In this situation, the motor control output (C9 - C12) would be at full value (like 2000) since the flight controller is trying to get more power from a motor that is slipping on the ESC control.
My next suspicion goes back to the flight controller being in some transition mode trying to reach the ARSPD_FBW_MIN setting of 15m/s. Why are C9 and C11 outputs sitting at such a low level when the planes is dropping from the sky?