I run into the same issue as this:
I have these arguments:
{
"post_type" : "plaatsen",
"post_parent" : "{{dc:post:id}}",
"posts_per_page" : -1
}
When I count the characters in the $args[‘post_parent’] it returns 14 characters exactly the same as the value in params: {{dc:post:id}}. So it’s counting the characters of the dynamic content tag instead of the value that is returned.