It seems that in some cases AI stops recognizing this betrothal and weds Roslin to someone else. Robb's betrothal is not broken in such case, leaving him unable to find another wife. Roslin is treated by the game as married, so there is no appropriate button in her diplomacy window that could cancel the marriage promise.
This most frequently happens when Walder Frey dies and Stevron becomes the Lord of the Twins.
This most frequently happens when Walder Frey dies and Stevron becomes the Lord of the Twins.