Hmm, I didn't see any of the characters grab the severed arm, but there's still a chance that someone grabbed it and is just out of sight. And if they did grab it, then, with magic Heal, there is a slight chance that it could possibly fuse the arm together.
That said, I like that they are actually seriously struggling with this fight, and that one even got her arm decapitated is even better, as it gives a stronger sense of danger, it's not just a chance that they'll get winded or a scrape at worst and defeat their enemies, no. Their very lives is really at stake here, so they need to really think tactfully and consider their situation on the fly.