We have made changes to how trial subscriptions are handled.
Now “Start Free Trial” will be displayed on payment button in Stripe pop-up. Also checkout result will show amount as 0.
In order to test that, you need to install testing version of the core plugin: https://s-plugins.com/testing-version/
Also update Subscriptions addon to the latest version (1.4.5) via WP Dashboard -> Updates.
And then make a purchase of your subscription product with trial in test mode.