Ossified tendons are sometimes found with Cretaceous bones, particularly Hadrosaurs. Muffin's bones are Jurasic, which is considerably older, and I have never encountered them in the prep work I have done. The bone is a bit too small for even a gastralia fragment. It is tiny compared to Muffin's ilium. It may have to remain a mystery.
The measurement files can be found at: https://zenodo.org/uploads/13287380