Leukemia's a blood cancer, so it does indeed spread. The issue is that your well-wish isn't exactly an orthodox one. I mean, let's be honest, it's not the best thing to say, "Hey, I hope your cancer doesn't spread and cause you more pain."
You of course know what you mean but to anyone else you're not exactly wishing them well. What you wrote isn't exactly optimistic, but rather overly realistic for the context. I know you meant well. It was just a bit of a faux pas.
EDIT: People may also misinterpret what you wrote as the patient spreading cancer to other people, insinuating that you bear some kind of ignorance toward what cancer is or something.