Similarity, my half PR was the first 13.1 miles of the Philadelphia Marathon and lasted for 2 years. I tend to race more 10 milers than HM, so never had a combination of fitness and “putting it together” in a half. Going into that marathon, my PR was over 69 minutes for the half, but I was tasked with pacing the marathon for the 2:18 B standard for as long as possible. We came through the half a bit faster than pace under 69 minutes and continued rolling for several more miles before I called it quits.
I counted this as my PR because (1) running and racing shouldn’t be serious to the point in which you’d penalize yourself because it didn’t happen during an official HM race and (2) it was an official split and a race director did accept the official split as entrance to a race field that required a 68 minute or faster HM for entry. Unless you ran a strange race that was total downhill for the first half before climbing the second half, I would absolutely count it — just as your “8 mile PR”, “9 mile PR”, etc. would have likely come during splits of other races.