It certainly seems to be a bug.  Here's a smaller example:
    type intopt = int option
    let foo ?flag () = let (_ : intopt) = flag in ()
prints as
    val foo : ?flag:<hidden> -> unit -> unit = <fun>

It still seems to be typed correctly:

# foo ~flag:17 ();;
- : unit = ()
# foo ~flag:"x" ();;
This expression has type string but is here used with type int
# let bar ?(flag=0) () = ();;
val bar : ?flag:int -> unit -> unit = <fun>
# [foo; bar];;
- : (?flag:<hidden> -> unit -> unit) list = [<fun>; <fun>]

The bug is also present when you run "ocamlc -i ..."

-- 
Eric Cooper             e c c @ c m u . e d u