Looks like he was rounding 3rd and went when he was laying on the ground. I don't think it was ever shown if completely stopped. However, the home plate ump very clearly was calling time well before he scored. By high school rules, yes, an ump can stop play with runners in motion if there is an injury he deems worth stopping for. However, I'm 99% sure that's not how MLB works. Like so many things in baseball, you can be upset at Muncy for faking all you want, but the umpires let it happen. Really, it's on them. To me, it's the same as when a batter calls time as the pitcher starts his motion. He can call time all he wants, but it's up to the ump to give it to him and almost all the time they do. Then everyone gets mad at the batter when really the ump is who you should be mad at.