Well, I'm still at a dead-end trying to reproduce this. Can you help by putting some temporary diagnostic code in wp-includes/post.php, in the function &get_post ?

The template function get_comments_number should be calling get_post for each video attachment. I need to know the result of the following call:

for one of the videos which is showing 0 comments inappropriately.

try echo( $_post->comment_count ); or print_r($_post);