How to analyze a "No GCS Heartbeat" event in the logs?

I amd using an mRo Pixracer with ArduPlane 3.8.4 and Mission Planner 1.3.56.

I was flying in manual mode when the airplane suddenly felt like it was not responding well. I glanced at my GCS and saw a “No GCS Heartbeat” alarm on the Mission Planner HUD and the flight mode was RTL… I have the system configured with the GCS Failsafe feature turned on so I know that it did its job well.

My RC control had a strong RSSI of about 70% and the airplane was about 300 feet away at the time. Also, the telemetry percent indicator in the upper right hand corner of the HUD showed about 99% at the time of, during, and immediately after the event.

On my RC transmitter I toggled between RTL and manual flight modes and the flight mode switched back to manual and the system did not go back into failsafe again. I landed the plane safely without further drama.

I believe that the system missed enough heartbeats to trigger the GCS Failsafe feature. I confirmed the “No GCS Heartbeat” alarm and RTL state by reviewing my PC screen recording of the flight. I can also see the RTL event in the DataFlash log and by replaying the telemetry log for the flight.

Can anyone recommend what to look for in the logs that might show exactly where and when the GCS missed missed the heartbeats? Are they tracked?