Something similar happened to me elk hunting fall of '18 in that I shot a bull that flopped "dead" at the impact. 350yds shooting kneeling off of a tall bipod. I've killed 8 elk with this rifle and a truckload of deer. Been shooting the same 200gn TSX load through it since '04. I watched for probably 15-20sec turned around grabbed my pack cleared the rifle's chamber and prepared to go get to work on my "dead" elk. When I turned back around, the bull was back on his feet so I quickly peel out of the pack, chamber another round, but the bull lays right back down as the other 2 bulls with him leave, it's obvious he's not going to (can't) follow. While he's down I can see his head/neck and nothing else. I decide that getting closer is a better option than shooting from my present position. I got into a shallow depression, got up to ~125yds and as I was settling in on the bipods for a precise killing shot he managed to effort back to his feet and begins to stumble away. I hit him again through the chest, cycle the bolt, manage to trip the floor plate (which I've never ever done unintentionally on any rifle), find a loaded round at my feet, chamber it, hit him a 3rd time quartering hard away now @ 150yds or so and he was down for good.
Upon breakdown it was pretty obvious the bullet pushed further than I'd expected and got him in the diaphragm/liver. No guts, but, I didn't miss them by much. In my case, the bull was out in the open and there was really no way for him to go that wouldn't take him either relatively close to me or run him past either my hunting partner or another party of hunters that were relatively close.
Alls well that ends well but there was a 5-7min stretch of uncertainty.
