What you did not mention was the prevailing winds at the time of the flight. I've seen pilots lose their drones in the ocean by not considering the impact of a headwind on the return.
You also can't overlook the obvious. The drone is not built to any standard that meets airworthiness or safety. The motors do fail and bring down aircraft regularly. I tend to only get reports about larger public safety drones but here is a similar incident with a Matrice 210. DJI Matrice 210 –...