I believe what I said was at times, pretty much most of the characters look off-model at times, not just Arthur himself. Like someone else previously pointed out, so far, Francine and Muffy seem to be the biggest victims of this; I'm not entirely sure just how, exactly and specifically, they're animating the show (again, I've animated in Flash before, so I know there's different ways of going about it), but I know most of the time, Flash can "auto fix" what it perceives to be mistakes, so I wonder if them trying to draw Francine and Muffy's faces the way they traditionally were were seen as errors in Flash that kept being fixed.
I really can't name any specific moments right now without looking at some of the episodes again, but as I recall, Arthur and D.W. both have had moments where they look just slightly off model, but again, Francine and Muffy stick out the most.