At the end of an auto mission the copter (3.6.10) switched to LAND mode instead of the RTL mode change from the mission. The only noteworthy thing is that at the time of the RTL waypoint the aircraft was above the SF11c max height (120m) and was receiving a constant 130m value from the rangefinder. I had configured the RNGFND_MAX_CM to be 12000cm. Terrain data was setup to be used during RTL and Land modes per the wiki.
My suspicion is that for some reason since it didn’t “know” where the ground was it ditched RTL and tried to just land.
This terrain-following wiki section has a warning but doesn’t say anything about LAND mode fallback.
The log shows Err: NAVIGATION-2 or NAVIGATION: FAILED TO SET DESTINATION