Two possibilites
BTDT on both possibilities!
One:, shot went to the left a tad, bullet passed between the right shoulder and chest cavity. I did that once with a .35reminton. Fortunatey I did connect with the humerous (leg bone)beaking it, and was able to kill the deer with a second shot. I've also made that shot and had deer run off and "get lost".
TWO: Possible with .243 and lighter rounds, though I had it happen with .35Rem also.
You hit the left shoulder (to the right of your dot on the picture) and bullet blew up on the shoulder bone. I had this happen this year with a doe I shot at the same angle, with a Rem 150gr Cor-lockt over 35.0gr of RL-15 for a chrono'd 2,400fps. Range was 27yds, measured. The exploding shoulder bone left a 2" x 4" wound next to the entry wound. Fortunately, the bullet deflected inward, lodging in the stomach after it took out lungs and liver. Deer ran 50yds and dropped.
My guess from 20+ years checking 100's of deer killed with various calibers, bullets, is that you either pulled the shot to the left and got a poor hit, or, the 100gr Power Pt. failed (I've seen that too!) and didn't give you a body penetrating shot. If the bullet blew up on the shoulder and deflected outward, the deer ran off on 3 legs, which they can do rather well!
I don't think it was the .243. Try using the 100gr Hornady Sft.Pt. Interlok or the Nosler 100gr Part. These are available in factory loadings too. My brother and nephew use the Hornady 100gr and have never lost a deer, nor required more than one shot. Most have been large bodied Montana mule deer or whitetails that have run over 200lbs. The 100gr Sierra's do real well too. I'm leery of the Winchester PowerPts after I nearly lost a deer in '81 shot with a 150gr from .30/06 at 3,000fps. Deer was well hit at range of 29yds. Bullet blew up and didn't penetrate.
(See, not just the .243 will "loose" deer with bullet blow-ups)
I've unexplicably lost deer with much heavier cartridges, and kill most of mine with .22 c.f.'s.
Thats why it's called "HUNTING", and not just "killing deer".
As long as the deer have a "say", its never a "done deal" till the deer is gutted!
I had a coworker that had a 180lb 10pt buck "come to life" in the back of his pickup 45mins later in town, at the court-house while he was showing the deer to some of the deputies. That .40cal 180gr SXT made a terrible hole in his pickup after penetrating that deer's neck! He had "neck-shot" the deer with an 87gr HP from a .25/06, and had a gaping hole in deer's neck. Only thing is, not much blood, and no bone hit! At least not until the pistol shot! Deer had been KO'd by the near head shot, but not killed. Darn near got out of that truck and got away! Like I said, it ain't a "done deal" till they're really dead!