The answer to that is, Marathon Infinity shipped with the wrong standard physics! Yup, they provided the M2 model by mistake. The correct model is attached, which you can also get by unpacking the Infinity map with Atque.
When I use Anvil, I have the same problem when I open the shapes file, but there's an option to "Create new Infinity file" which solves the problem. Don't konw if shapefusion has that feature.
The Infinity merge file has physics merged in, and therefore some levels (like Big Time) have their own special physics. So the bundled physics actually has no effect on Infinity itself.
Unlike Marathon 2, which did not have physics merged in; you can load in your own physics file and play through the entirety of Durandal with it.