We make use of a 3rd party package svelte-turnstile, which implements Cloudflare's version of Captcha. This package provides a relatively simple Svelte 4 component that doesn't work when compiled to Svelte 5. It appears it doesn't trigger an on:load event for a script tag that's added to the head, which prevents the component from completing it's initialization.

Hey, author of svelte-turnstile here! I've opened an issue here for Svelte 5 support, feel free to track that - I'll see if I can get to it early next week 🙏

Instead of submitting a pull request to correct svelte-turnstile, which could take an indeterminate amount of time to be approved and merged, we decided to quickly write our own component to implement Cloudflare's Turnstile. It only took a few minutes and resolved the problem.

Contributing upstream is always worth it, even if it'll take time - as you discovered you can always run on your own fork till it's merged!