When spawning a vehicle, the auto-save will occasionally freeze the game. If it does recover, the creation will sometimes disappear. One does not always lead to the other. It will still be on the map, but trying to recover it fails to load it back in and it remains on the map. I was able to reproduce this on a creative map without any add-ons. I was testing and fixing a rocketship, so it's possible in one instance the previous save had not fully completed when I tried to again spawn the vehicle. I had to force-close the game. Might be possible to reproduce by quickly spawning, returning and spawning the vehicle several times.

Side note, possibly unrelated and caused by an environment mod, during one instance the fuel and resource tanks disappeared as well. Reloading the save did not fix it.