Find the skill that the Gheists use in-game to knock people over. Think it might be called 'Force Push' (don't quote me on that) under maybe the cone skills. It's surface damage type (or similar property name) is set to 'sentinel' I believe. I think for some of the skills to properly work you need to have an entry for it and removing results in no damage. The sentinel doesn't actually cast a surface, but may fix this. But this may also accidentally remove surfaces instead, so check that too.
I suggest restarting the editor and/or loading up the sandbox and then reloading fort joy. Had this problem myself a couple of times and that worked.
