You see the absence of an arm. I don't think you actually see it come off. You don't see the disembodied arm. And you don't really see much but a shock of red and the hint of a stump on his coat.
Seriously, I don't think Muschietti actually shows you much of much there.
It's kinda like how everyone swore they saw some crazy shit in the napkin during that one shot in Blair Witch Project.
Yeah they really didn't show much. Pennywise gets a big ol chomp on his arm and then the arm is just gone when it shows George fall back. There's some puddles of blood on the ground but that's about it. Wasn't as gruesome as people made it seem.