Dog Pooping in Grass Not Where Trained to Go?

I have a 2 year old Lab who's been trained to poop in a marked off dog run area. Suddenly he is pooping also in the grass for the first time in at least 18 months. We can't think of any variables that have changed so we can't figure this out. We do a good job of keeping the dog run area picked up of his poop, putting down new pine straw, pulling any weeds in that area, etc.

But for reasons we can figure out, about a month or two ago he started pooping 50-70% of the time still on the pine straw in the dog run and suddenly 30-50% of the time in different areas in the grass around the yard. When in our yard he still only ever pees in the pine straw (always in the exact same spot of the dog run), so it's only the pooping that's changed.

We've consistently picked up the poop piles in those spots in the grass then watered them down and sprayed that area with some vinegar so it doesn't smell like dog poop and seem like a marker for the correct place to do it again. Like we did 1.5-2 years ago while training him as a puppy, if we see him circling and about to poop in the grass we'll tell him 'No' and he'll run to the dog run area and do it there (or finish there if we didn't see him in time and he'd already dropped some in the grass), but those haven't stopped him doing it like it did while we were at the 'tail end' of the training process with him as a puppy. Any ideas?