Did you try verifying local files for the game?
Will the entire party go up or down stairs, hatches, etc, if they are chained together?
Did you try starting a multiplayer game, rather than single player, to see if that would make a difference? (it shouldn't, but it couldn't hurt to try).
If you are still having this problem, and have not done so already, you can email supportdos@larian.com, with a description of the problem and the report.zip file generated by the
D:OS support tool.