Yeah, except it didn't. After Axel (or any Organization member for that matter) faded, no Heartless appeared afterwards. I think, based on what we've seen so far, it's safe to assume that if a Heartless doesn't appear around the area shortly after a person lost their heart, a Heartless wasn't created at all.
Weeell, there's Isa, if that counts at all. It is kinda funny to think that Lea's sort of befriended the same person at least three times now.
All I'm saying is that Axel had a body and then he grew a heart. Once he was destroyed that heart had to be released and that's how heartless/nobodies are formed. However, because Axel's body wasn't his own it would not create another nobody. That body would just be returned to Lea, but what about the heartless?