Complete loss of elevation and throttle authority (Ardupilot v4.3.3)

Flying in cruise at 31m/s starts an unplanned descent. No significant pitch response and engine continues at 100% despite zero throttle.

Once back to FBWA it was resolved.

Could this be caused by an error in TECS_PTCH_FF_K that instead of -0.06 uses -0.6?