KSTrapper...great determination. My own experience with this shot is not good either. There was a recent article in one of the bowhunting magazines which basically told hunters to move their aiming point a little further back and then stated it was good for several inches in all directions. I remember my gut reaction to that article was, not in my experience, but I don't have time to dwell on stuff when the season's rolling.
Maybe 15-20 years ago I hit one similarly to you, although it was a pretty sharp downward angle. Figured it was a down-and-outer, waited 30 min., got on the trail, jumped it, and later trailed it 600+ yards and never found it. It was killed two weeks later in rifle season, with no obvious wound to tip the hunter off. (The buck was a one-of-a-kind rack; no doubt it was the same buck).
A couple winters ago I hit one very similar (behind the heart, below the upward arching lungs). This doe ran off hard-hit and acted like she was going down any second as she topped a little mini-ridge 40 yards away. I waited just a few minutes and thought I'd take care of this before dark, and got on the trail and I'll be darned if she wasn't standing on the other side of that ridge in real distress. Me pushing her probably caused her to stress out that one lung she had left but it was 15-20 min. after the shot. She ran about 30 yards and laid down and died.
Then this year, I wounded and lost a buck hit low and behind the heart (very low, though, maybe only a few inches above the bottom line of the deer). Quite a bit of blood whenever he stopped, but eventually petered out and despite LOTS of searching, no animal. He showed up on trail camera after this event but it was inconclusive as the photo didn't show the hit area.
And also this year, my second buck was hit much the same but as it turned out, about halfway up the body line. Good blood, but despite waiting 90 min. and then stalking the trail, 100+ yards into a fair trail, no buck. I backed out, came back 3-4 hours later, and after a very difficult 100-150 more yards, found the buck. Liver, part of one lung maybe (I was in a hurry). I'm sure if I'd pushed him at all early, I'd never found him as there was not a great deal of blood the last 1/2 of the trail.
I'm just not liking that low and back shot at all. Again, congrats KSTrapper on your determination and I'm glad you found the deer. And remember that the little critters need to eat too.