JB,I witnessed almost the same thing on a WT buck last week.
My brother shooting a 243 with 85 grain TSX's that I load for him, shot a buck at a ranged 270 yards. At the shot, the buck showed zero reaction. It sprinted 150 yards, up a slight hill and into the timber and out of sight.
My initial thought was it was a clean miss. My brother said the shot felt good except for the fact that the buck had just started to step forward when the trigger broke.
We had perfect snow for tracking and went over to take a look. Zero sign of any hit and anything at all would have been easy to see. We followed the tracks into the timber 75 yards and still nothing. By now we had probably gone close to 250 yards and saw nothing to indicate a hit.
We went on another 20 yards in the heavy timber and there lay a very dead buck. The bullet had just knicked the very back of the lungs a bit above center chest and exited.
I fairly new with using the 85 TSX and have never seen a deer make it this far with lung damage. Part of me wonders if using a bullet that fragmented more would have done more damage on this fringe hit and shortened the trailing. On dry ground, with no snow, I'm afraid this buck would have been a tough recovery.
