Game breaking bug in A Cockalorum mission (A World Without Gold)

You can see the bug in the last 10 minutes of my stream here:

https://youtu.be/2vTo94b0SKg

The mission says to sink all the ships, and I do, but then after I finish sinking everything, I get a mission marker 17.5 KM away, far off the map (I estimate it would be somewhere like the top of Florida or Alabama or something if that were on the map). The mission locks me to the combat area, so I can't leave the area, and I can't seem to quit the mission, and reloading my autosave and reloading the game itself doesn't solve it either.