Losing a deer is a truly awful feeling that one has to experience in order to understand. I've lost two, that I can recall, when using a centerfire rifle. But I've never lost, fortunately, a deer shot with a muzzleloader. While sitting on my stand years ago I had a buck trailing a doe cross in front of my position. I fired and he dropped. When I examined him I noticed 3, two huge and one small, scar tissue lesions high on the back and on the ribcage. They were healed but still looked nasty. When I dressed the deer out it became obvious to me what had occurred. At some point, possibly even the archery season which was earlier in the fall, he'd been shot with an arrow which hit and mostly skidded just under the skin without actually penetrating the body cavity. The arrow had gone through with little effect and exited.
I have on my wall at home a nice, heavy 9 pt. rack I cut off a deer I found where I was hunting. Coyotes had been at him and decay was advanced. Someone had shot this fine buck and lost him, probably from a poor hit. I salvaged the rack and the coyotes got the rest.