I've fallen under the map a few times when I spawn on the Turquoise Islands. It looks like the objects don't load fast enough.
It sometimes happens with vehicles too, they fall under the map.
I've experienced this a couple of times as well. It might help to freeze the player for a second or two immediately after he/she spawns, so that objects stream in the player's screen.
Besides, custom objects constituting the FC airport don't seem to stream anymore. I'd call this a minor inconvenience as planes can still be taken off.