Had the same problem. Stopped the Highwaymen before they could destroy the East or West vents. Then the music just kept playing. I ended up blowing up the two targets myself and it proceeded to the next part where they were trying to plant a bomb. I successfully defended and completed the mission that way. Maybe Ubisoft underestimated the players defending the first part and didn't think we were capable of defeating them before they moved to trying to plant a bomb?
I'm stuck too. Just finished killing off the highwaymen while defending the east and west posts. Music is still playing and exploding barrels are still being thrown.
Can confirm there are no guys stuck anywhere. I guess I'm going to destroy the targets myself as suggested in another post so it advances the storyline.