Sometimes on large or older maps strange things can happen with props. Typically, the problem is invisible walls that linger even after their prop's deletion. Far less commonly, I've seen oddities with holes in collision meshes before, but replacing the prop tends to fix it.
What makes this case interesting is that a default barrier failed, which I've only seen once before. (I found a massive 32x 20m barrier in the middle of some random map that affected nothing.) If replacing the prop and adding in another barrier both fail, perhaps that means a specific area of the map is corrupting collision mesh placed inside it, and you'll have to circumnavigate that area.
What makes this case interesting is that a default barrier failed, which I've only seen once before. (I found a massive 32x 20m barrier in the middle of some random map that affected nothing.) If replacing the prop and adding in another barrier both fail, perhaps that means a specific area of the map is corrupting collision mesh placed inside it, and you'll have to circumnavigate that area.