As your code looks quite generic, the obvious answer and thing to debug is that either the if condition is not true when you’re expecting it to be, or $media_content isn’t getting generated as you expect.
Are you 100% sure that on all the other sites the post_type is 'my_custom_post_type' when you expect it to be?
You can debug things like this by temporarily inserting code like:
function my_custom_post_type_the_post($post_object)
{
$post_object->post_content = "debug: " . ( $post_object->post_type == 'my_custom_post_type' ? "yes" : "no" ) . $post_object->post_content;
$media_content_test = generate_media_content( ... ); // make this stuff self contained so it's easy to test
$post_object->post_content = "debug: " . $media_content_test . $post_object->post_content;
// original code below
// The post object is passed to this hook by reference so there is no need to return a value.
if(
This way you know things like:
- The hook definitely ran?
- The conditions you were expecting are actually true or not
- The output you were expecting is getting generated properly