all-defined-out doesn't seem to work well with macro

When running the following a.rkt, I get the expected result:

#lang racket

(provide module++)
(require syntax/parse/define)

(define-simple-macro (module++ name base xs ...)
  (module name base
    (provide (all-defined-out))
    xs ...))

(module++ bar racket/base
          (define a 2))

(require 'bar)
a ;=> 2

However, running b.rkt, which uses module++ in a.rkt, fails.

#lang racket

(require my/a)

(module++ foo racket/base
          1)

provide: identifier's binding is ambiguous
  context...:
  matching binding...:
  matching binding...:
  matching binding...: in: provide

Reproduce the difference between a.rkt, which uses module++ locally, and b.rkt, which requires my/a before using module++; inspect the syntax/parse/define macro and the generated module's provide (all-defined-out). Done means the module++ expansion can be used from another module without the ambiguous-binding error.