In D2:ED a couple people that left bandits outside (so the cutscene didn't get triggered) were able to run straight back to the elevator (which was disabled until after the cutscene). Maybe in DKS it starts off with that part of the tunnel already caved-in, but you should still have gotten a cutscene.
Did you check all of the cells? Even if you didn't get a cutscene with the prisoner as you approached, you may still be able to trigger it by opening the cell door. The cell with the hidden passage is to the left of the one with the busted in door, IIRC. The first wall can be removed with a button on a vertical wooden beam on the left, and the next by a similar button on the right IIRC facing the movable wall.
If you return to Broken Valley, maybe finish off anything you have left there, and then come back to Sentinel Island, that might reset something. On the way back maybe you could double check if there are any bandits outside.
Some people have had a problem with missing or invisible opponents. That, or something similar, could be causing your issue. Try emailing support@larian.com, with a description of the problem, and maybe a link to this topic. With your save (see Lar's post
here about how to send them an Xbox save) hopefully they can figure out what the problem is.
The UK Xbox patch should be ready to submit to MS at the end of this week, after which it will take a couple weeks to be certified.