Webhook - "attachment add" is not triggered in special situation

Hello,
I have created two webhooks, with the following filters:

Filter 1

"filters": [
      {
        "action": "added",
        "resource_type": "attachment"
      }
    ],

and Filter 2

   "filters": [
      {
        "action": "added",
        "resource_subtype": "comment_added",
        "resource_type": "story"
      }
    ],

Both webhooks are triggered, just fine when a new comment is created, with an attachment, and sent right away.

But when a comment is created, with an attachment, and sent after some delay, the first webhook is not triggered. The situation is as follows:
New comment is created and an attachment is added
User leaves the comment unsent
The current intermediate state of the comment is saved
After some time (more then one minute delay), the user sends the comment.
The first webhook (attachment) is not triggered, but the second webhook (comment_added) is trigged just fine.

Is this correct behavior? I couldn’t find any information regarding this situation.

Thanks.

Sincerely,
Tom Sladek