RouteValueDictionary overridden by context

Question

If you have a route:

routes.MapRoute("search", "{controller}/{action}/{filter1}/{filter2}/{filter3}", _
  New With {.filter1 = "", .filter2 = "", .filter3 = ""})

then in a view satisfied by the route pattern with a url of /member/search/dev/phil/hoy, when you attempt to create another route url with only filter1 present i.e.

<%=Url.RouteUrl(New RouteValueDictionary(
  New With {.controller="member",.action="search", .filter1="dev"}))%>

the result is the current route /member/search/dev/phil/hoy, not the expected trimmed route /member/search/dev

I have managed to work round the issue by using RouteTable.Routes.GetVirtualPath method directly, but does anyone know why it works this way or is it perhaps a bug?