Output to STDOUT blocks reporting to STDERR

Given a Gren file with a known compilation issue like so:

module Broken exposing (..)

main =
    Html.text "Hello"

If I compile this module with output to a file, compilation errors are reported to STDERR:

$ gren make Broken --output=broken.js --report=json 2> errors.json
$ cat errors.json | jq
{
  "type": "compile-errors",
  "errors": [
    {
      "path":
# etc...

But if I redirect the output to STDOUT, reporting on STDERR disappears.

$ gren make Broken --output=/dev/stdout --report=json 2> errors.json
$ cat errors.json
# file is empty

Gren version 0.6.6 on Linux.

No source file or test is named. Start by reproducing both gren make commands from the issue on Linux, then trace how --output=/dev/stdout affects compilation errors and --report=json output. Done means JSON compilation errors are still written to STDERR when compiler output is sent to STDOUT.