I have experienced this before the patch, and it is not a bug. As already posted by Remus, a lack of bridge means that none of the provinces that border or encompase the bridge have roads yet built. Armies are not getting caught on these, they are placing them there because they are easily defensible positions. Instead of a bridge, there is a ford there. So if you were to attack this army, you would fight in a battle similar to that of a bridge battle, except instead of a narrow bridge crossing, you have a wider ford in which to attack or defend.