Dynamic lightning - suddenly player can see everything

During our last session one player who joined the session a bit later was able to see the entire map rather than just the places where he had vision himself. When I marked his Token and pressed ctrl+L I could only see what he was supposed to see. I asked him to relog Roll20 and then everything was shown to him as it should. I don't know what caused this bug, but I would like to avoid it happening again. Anyone have an idea what happened here? My best guess is that it was related to me as GM looking at his character sheet and controlling his token until he joined.

Sometimes players with slower connections, or not enough bandwidth to cover the various other things they might be doing at that time, fail to load the DL layer, its happened to my players a couple of times and a simple refresh always solves it... failing that you could switch them to another page and back and that would probably do it. It's a pain but other than using fog of war to make sure everything is covered 'just in case' there isn't a lot more than you can do other than hope your players are honest...