Tbh, I can believe it - Diacetyl is a notorious problem with hoppy beers and one that can often only rear it's head when the beer has got warm during transit.
I've judged 3 forum comps, 2 of which were American Pales and every time I've had beers with diacetyl.
I'm not immune to it either - I've had it happen in a BJCP comp and I've had a whole batch go down the drain. That one took about 2 weeks for the diacetyl to show up in the keg so would have been easy to send it off to a comp without knowing there was an issue.