Hello there,
Thank you for reaching out to us.
I can tell you that the [mailpoet_page] shortcode is working as expected currently.
If you preview your custom Unsubscribe page on the Edit Page screen, the shortcode won’t render i.e When you create your customized unsubscribe page you can’t test it in the preview mode like you would test all your other pages before they go live. This is why you are seeing the shortcode.
After having your new unsubscribe pages created and published, you will need to access MailPoet > Settings > Basics.
On the Basics tab, go to the Unsubscribe page section and select the custom Confirmation and Success page created from the drop-down menu: See sample screenshot below:

Link to image: https://d.pr/i/6NNrsG
You can click on the Preview link to see what your custom Unsubscribe page will look like. Don’t forget to save your settings when you finished editing. Please use the Preview buttons above (in the screenshot above) or test with Unsubscribe links in actual emails instead.
I hope this helps.
Have a great day.