Most people including me have some ankle issues after the fibula free flap. They are not major but noticeable. It hurts mildly when I walk a lot some days. If you think about it what remains of the fibula down there is just hanging out in space, so the small amount of rotational torque it prevents when it ran all the way to your knee area isn’t there any longer. This isn’t dangerous unless you are doing some extreme loading of it. That should not happen in normal peoples lives. I’ve never heard of a person have a second surgery to stabilize it. You are missing a bone that was there for a purpose, a minor one, but designed into your leg by evolution to be there. You should expect it not to be perfect, but completely safe to live with in my opinion thinking of the hundred or so people who over a few decades I’ve seen have this done.