post_parent is 0 when uploading new files

I have a custom-type post I'm trying to attach some audio/wav files to using the wp-admin UI.

When I upload the new files using the Add Media popup those get saved and embedded into the post just fine, but in the database those attachment-posts have parent_post set to 0.

It records the post_date and post_author correctly, but not the post_parent.

What is the reason for that behavior and how can I fix it?

Update ok so this only happens to the posts with custom post-type. When I upload the same audio file to the normal post it gets attached properly. If I try disabling the plugins it will remove that custom post type I'm working with and I won't be able to test.

Update2 this happens during the upload process. Here's the var_dump($_REQUEST); output from wp-admin/async-upload.php when uploading an attachment to the regular post:

array(4) {
  ["name"]=>
  string(42) "my_file.wav"
  ["action"]=>
  string(17) "upload-attachment"
  ["_wpnonce"]=>
  string(10) "457d8c9ee7"
  ["post_id"]=>
  string(4) "1234"
}

The post_id key is missing when I'm uploading an attachment to the custom post.