I am trying to use proper REST urls with MVC. To do that I switched default Routing from:
{controller}/{action}/{id}
I was having a similar problem. Route values that were passed to my controller action were being reused when I tried to redirect the user with RedirectToAction, even if I didn't specify them in the new RouteValueDictionary. The solution that I came up with (after reading counsellorben's post) with was to clear out the RouteData for the current request. That way, I could stop MVC from merging route values that I didn't specify.
So, in your situation maybe you could do something like this:
[CustomAuthorize]
[HttpGet]
public ActionResult Approve(int id)
{
_customerService.Approve(id);
this.RouteData.Values.Clear(); //clear out current route values
return RedirectToAction("Search"); //Goes to bad url
}