Does not hide other shipping methods

I encountered this problem. Some testing showed that if you add an item to cart and see both free and charged shipping, then activate the plugin, both free and charged shipping will still show as you say. But if you empty the cart, activate the plugin, then add the same item to the cart – this time the charged shipping is not offered, so the plugin is working

I found the same to be true of the snippet:
https://docs.woocommerce.com/document/hide-other-shipping-methods-when-free-shipping-is-available/

Your issue may be different.