How to Add custom subtitle as og:title to Facebook Open Graph Meta Data in WordPress Themes?

It could be that there are duplicate Open Graph meta tags being generated AFTER your special hook. Priority would be given to the later tags, which means your customizations don’t happen.

Try setting the order of your hook later, so that it happens after whatever else might be generated:

// default is 10; I'm using 11 arbitrarily
add_action( 'wp_head', 'insert_fb_in_head', 11 ); 

More importantly, though, you need to confirm nothing else is generating OG. Check your plugins, and consider using on that easily lets you customize OG data as you need, like Complete Open Graph:

https://wordpress.org/plugins/complete-open-graph/