This first happened to me when I was trying to make a simple "drop me off" kind of scenario with a boat. I put the boat clearly in the water, but it kept spawning the boat up in the rocks on shore killing my driver. after moving it more and more out to sea I noticed the editor kept spawning the boat in the same spot up in the rocks. I got frustrated and found a different way to do my mission. then I tried a convoy scenario getting hit by an IED, the first two trucks spawned fine but the third in the convoy glitched just off the road in the trees and was unable to move. even my truck was in the bushes a bit but followed the first truck fine.