My husband and I were playing multiplayer (Steam, PC) and both of our buggies disappeared within a couple seconds of each other. I was streaming at the time (video is here) so I can show you the footage. At 1:14 into the stream you see me park my buggy and go down into the cave. Around 1:16 the game starts acting really funny (my terrain tool kept going even after I released click, then I suddenly died for no apparent reason at 1:17). At 1:18 you see my buggy still where I parked it, and my husband's buggy at the bottom of the next ramp. At 1:24 you see my husband's rover still parked at the bottom of the ramp, but a few seconds later I reach the top of the ramp and mine is gone (I didn't realize at first). At 1:25 I peer down and my husband's has now disappeared as well. We checked and they were not back at the base, nor were they in the ground where they had disappeared. :( They seem to have just vanished.
Stream Recording: https://www.twitch.tv/videos/456502093