Contracts not enforced by scribble/manual

The following buggy program gives an error message blaming scribble/private/manual-class. Instead, scribble/manual (or perhaps wherever box-splice comes from) should enforce contracts and blame the buggy module, which would also generate a more comprehensible error message.

#lang scribble/manual

@definterface[example<%> ()]{
 @defmethod[(example) any]{
  @#'"A syntax object is not a pre-flow"
}}

Error message:

decode-flow: contract violation
  expected: pre-flow?
  given: (box-splice (list (nested-flow (style 'vertical-inset '()) (list (table (style 'boxed (list (attributes '((class . "RBoxed"))))) (list (list (nested-flow (style #f '()) (list (nested-flow (style "RBackgroundLabel" (list 'decorative 'command (alt-t...
  in: an element of
      the 1st argument of
      (-> (listof pre-flow?) (listof block?))
  contract from: 
      <pkgs>/scribble-lib/scribble/decode.rkt
  blaming: <pkgs>/scribble-lib/scribble/private/manual-class.rkt
   (assuming the contract is correct)
  at: <pkgs>/scribble-lib/scribble/decode.rkt:71.2

Run the supplied #lang scribble/manual reproducer and inspect scribble/decode.rkt at 71.2, then trace the contract boundary into scribble/private/manual-class.rkt and the box-splice path. Done means the same program reports the buggy module rather than manual-class, with a more comprehensible contract error.