Actually, vanishing away teams is different from the one where your away team just falls through the ground. This one they actually still follow you and actively engages enemies, only any enemie they engage will also disappear into their realm preventing you from targetting them. They still die and drop loot back into your realm however. Only happened to me once after some patch a while back ago.
Falling through the ground however, is quite a bit more common, and does happen quite a bit on medical crate missions.