Does this mean a game started as Singleplayer / Offline can be turned into a MP game in-game?
In Divinity : OS? Yes, because there is no singleplayer/multiplayer mode. There is only drop-in/drop-out, and whether you as the player are actively allowing it or not at any given moment.
In that case that is a massive bug then. ;/
At least when I make a campaign for this, I want to be able to define whether it's SP or MP/COOP and enforce it.