This is a vanilla mesh, but it is not used in the original game, so I don't know if this is something that should be fixed by the Patch for Purists or not. Morrowind Optimization Patch also covers some unused meshes, but that hasn't been updated in a long time. Anyway, this mesh is used in a couple of places in TR, so the issue mainly exists there (and in other mods that use this mesh). The issue is that it has a buggy collision mesh, which doesn't extend all the way to the upper edge, where it is usually connected to a bridge section, creating a hole in the floor that the player or NPCs can fall into. It's not a big hole and you can get over it, but it's still there.

I've uploaded a fixed version (also with improved UVs) to the Nexus. In the screenshot, the visible geometry is transparent to illustrate the problem.