In the A Last Stand mission, you can clear maybe a fifth of the map before you run out of exit points. There is a coward villager npc in the NorthEast who won't lower a bridge for you, and at the SouthEast limit you have a bridge marked on the map but it's unassembled and impassible.

He's supposed to do so if you speak to him. At least he did in my case. Try clearing the whole map accessible at this point.

I just finished the game, I had the same problem, it's definitely a bug. Reloaded a couple of times the same mission just before talking to the coward but it didn't help, the coward wouldn't lower the bridge. I relaunched the game and reloaded the mission and the coward lowered the bridge this time.

Also in the map where the catapult destroyed the city portal I couldn't go through portal rubbles, reloading made the rubble disappear and I could go through.

Quick-saving often (F6) is a must.