I also played around with the speeds and came to the same conclusion that it might have had a forced landing near the home point in case the drone really tried to complete the mission with the 11.2 km/h from waypoint 10. But usually I regained signal around waypoint 19 and 20 on the previous missions. I also searched this area already though. Might go on another walk along the waypoint track next days. The snow melted now and some wind kicked in, maybe it fell off of some tree now.
Is it actually a thing that when “Enable DJI’s Gimbal Interpolation in Waypoint mode” is enabled, the Spark might not correctly adjust speed for different waypoints when out of signal range? Before I enabled this option I never had a warning before I took off, saying something like “beware you manually adjusted speed for some waypooints”, even though I manually adjusted speeds for different waypoints before.