How do engineers make sure UAV don't land on their front propeller?
Due to center of gravity limitations, it would be impossible for any type of aircraft to be loaded so that the front became the heaviest part. The aircraft would not be able to fly.

These UAVs are meant to crash land and DO land on the front propellor. You can do a Google search and see multiple videos showing the “break apart” landings they are designed to do.

Here are some landing videos: 

Puma landing 2 

Puma landing 1

Coast Guard Conducts First Unmanned Aircraft System Deck Landings