From your description of the load and the shot I'd guess the bullet either hit the shoulder bone and fragmented or went through the muscle above the spine. The muscle shot should have cause a lot of bleeding, but with the rain????
The 300 grXTPs has killed the only two I ever shot with them, but both of those shots were right into the heart/lung area. I think of the XTP as a "fragmenting" bullet at muzzleloading rifle velocities and place it to avoid anything more than ribs when it comes to bones. For a shoulder shot one of the bonded bullets or an all lead conical would give much better results. I like the Barnes Expander MZ as my #1 choice, but unfortunately not every rifle likes them. The only reason I was using XTPs this year was that the inline I had didn't shoot the Barnes nearly as accurately as the XTP. I hope (if I keep that rifle until next season) to find a sabot that will fit the Barnes better in that bore and give good groups.