Confirmed for 1.13-pre6
Who's saying anything about the Passengers tag? I'm talking about the RootVehicle tag of the players. Players should store the entity they are riding in their RootVehicle tag right? Well that tag doesn't exist if there are 2 players in the boat, which make no sense to me.
Yeah I think you are right, as I just noticed this "bug" also occurs in 1.12.2.
It's a shame though, because I think everyone will expect this to just work.
😞