Mission Plan To Lower Altitude

Just had an experience (crash) which baffles me. It was an Auto Mode mission starting from my backyard, going up to 40m to avoid some low hills, flying out to a lake which is about 65m lower than my yard, dropping to 15m for a Loiter circle to shoot some video, then rising back to 40m, and home. It never made it home, and I found it had crashed into one of the low hills on its way back. I was able to retrieve it and get the log (here attached). I have a lidar unit installed in the belly that gives distance about ground readings up to 20m.

Per the log of BARO Altitude, (pic attached) it behaved as expected until it got to the lake. It then descended to 40m below starting elevation (a descent of 80m which would put it about 15m above the lake), did it’s circles to shoot video. Then unfortunately, it came up only 40m and headed home. Not high enough to avoid the low hills.

My questions are:
How did it go down to 15m above the lake since per the log the lidar RNFD Dist seems valid only at the very beginning and end of the flight? (It was getting occasional valid lidar readings)

Why did it seem to reset the base elevation and not come back up to 40m above home?

How should I have set up the waypoint altitudes?

Any ideas?
Log of Flight
Pic of Altitude Graph

Anybody there? Opinions? Clarifications?

@Mustafa_Gokce may be he can solve your query can you tell if i can go to negitive altitude i want to dive my plane into water like 3 feets underwater so is it possible that i can do it, what happens my plane just trigger rtl mode when come here groundd any other solution