Manual add_to_cart() not working in 2.3.x (worked in 2.2.x)

After upgrading from Woocommerce 2.2.1.1 to the latest version, this line $woocommerce->cart->add_to_cart( $post_id ); in a plugin file fails and hangs. I have checked the cart manually and although the cart is emptied, it isn’t subsequently updated with the correct product.

This is the chunk of code:

global $woocommerce; [Works]
$woocommerce->cart->empty_cart(); [Works]
$woocommerce->cart->add_to_cart( $post_id ); [Fails & Hangs]

Any suggestions welcome!

https://www.ads-software.com/plugins/woocommerce/