Scython said:
Perhaps it's an easy solution because they use close-up panels in the manga to point out what's causing the constant healing. For them it's probably less obvious.
Also, that whole 'closing the wound' part made no sense. Like... why would you even consider shooting his leg when you know that it will cause an explosion? I don't even get why it stops the bleeding at all but okay. If they could mostly stop the bleeding on his arm with a simple wrap, why not do the same with his leg?The leg kept bleeding because of the acid, by removing the stump he also removed the acid, and they can now patch the leg up.