If image was uploaded to Media Library and inserted into the post, its attachment record has that post ID as a post_parent.
But if user just select existing image from Media Library to insert to the post B, its parent post A stays the same, older one. URE can not select/count such image as attachment to the post B, which is allowed for editing by current user. URE looks at this Media Library item as attachment to the post A.
Please open one of the child pages allowed for editing, click “Isert media”, upload new image and insert it into the page. Then open “Media Library” under restricted user. He should see, such image. But he will not see images uploaded by others for posts/pages, which he can not edit.