My apologies, small and low poly is a misnomer since I can’t see the geometry and it seems like part of a giant wall or background art. More accurately what I meant is that I wouldn’t personally use Nanite for this structure since the shape is simple and you’re already using textures to fake depth. Unless the player is climbing it up close and personal the geometry could be rather simple.
To refocus on your issue though; It seems the collision hull generation for Nanite is definitely bugged. I can replicate a similar issue where the hull is inaccurate for shapes with more complex (like convex points). Might be worth the bug report in the meantime: