Obligatory YouTube Video:
Here's what happened: The copter was heading to the northwest waypoint at 25 meters altitude.
Next it turned to the southwest heading to the next waypoint.
The altitude started dropping, and the NavThrottle was increasing as normal, clear up to 182.
The altitude was still dropping, the NavThrottle Suddenly took a dive to 55, and started up from there.
In the meantime the copter was dropping....
Next the NavThrottle integrated to 114, and then remained flat, as the altitude dropped even farther, and I almost hit a tree, as seen in the YouTube Video. Fortunately, I went to Stabilize, and got my copter back (after another mishap as seen in the log, which appears to be my fault!).
Darn, another bug? a Nav Throttle Setting?
Screenshot of APMLogVisualizerGraph is attached, as well as TLOG and LOG files