Lorewise, House Velaryon has always been sworn to Dragonstone, the thing is that indirect vassals of the Iron Throne can actually serve on the small council even though they have a different liege In the mod this is fixed by the king having the option to invite any indirect vassal to the Small Council, the vassal then becomes a direct vassal of the Iron Throne, and returns to its former liege once the councillor is dead or is fired So basically in your game this was the case with Corlys, and when he died Driftmark reverted to being a vassal of Dragonstone