Not able to follow mission altitude

We have been flying quadplane for surveying mission. We had given multiple flying altitudes during the mission ranging from 140-200 meters above terrain. The craft initially maintained the desired altitude but then kept on flying at 120 meters (relative) instead of 200 meters(terrain) in the end of the mission. I am not able to troubleshoot the reason for the same.

Bin file of this flight.

Everything is correct in the configuration of the height maintenance.

In the mission definition did you indicate heights relative to terrain?

The mission plan was in terrain mode. The altitude commanded was 200 meters above the terrain and it was flying in relative mode at 120 meters.

It would help if you could post the log.

I did post the log in my first post itself. I guess you did not notice it. Let me post it again.

Is this a new behaviour on an aircraft with lots of flights or is it a new aircraft?

It has done a good number of flights. Before this and after this flight. This particular one was an issue. I am worried about the future if it happens again. So knowing errors and rectifying them is important. But it seems not a lot of people are interested in solving/knowing about the issue.

So finally this time around the bird stopped following terrain after few waypoints and crashed into a mountain. Still not able to figure out what was the reason. I checked that the ter_pend was 0. The mission was in terrain. The desired pitch vs actual pitch was also spot on. Yet it kept on loosing altitude. Please look at the crash folder in the link below for the recent crash.

In the tlog I observed 1 more thing whereby the target altitude is increasing but the bird kept on loosing altitude instead.

Is there any update on this?