Issue was caused is MB from version 5.4.7. Downgrading fixed the issue.
Thanks so much Anh, sometimes one does not see the forrest for the trees.
I did not enable the required AIO plugins on the subsite.
Best regards!
Okay the script is working if I include it directly to the enqueue_script JS. But it's not working from the template.php with <script> tag.
Okay the trick is to not serialize the value when using add_post_meta as it will be serialized through that function.
Problem is solved.