Two problems have been recurring for me the last few days in Pantheon (don't play anything else).
First, when it finds everyone has connected and loads, about 1/5th of the time I hear the "player has left" and when I get in the game, everyone have disconnected from me and I am now facing bots. This happens as the game has just finished loading the map. Either something happens at this time, or the connections weren't as stable as they should be.
Secondly, even though the game is supposed to be P2P to avoid such a problem as this, sometimes when one player leaves or is disconnected, everyone else will drop as well. It's not me leaving, and yet the game treats it as such. I know I didn't REALLY get disconnected, because these guys often say something like GG before they leave.