kit: Netlify Adapter: Netlify Forms don't work

Describe the bug Netlify Forms don’t work in a Svelte component if setup with this approach and using the Netlify adapter.

_Edit: For a workaround/fix see the comment below https://github.com/sveltejs/kit/issues/942#issuecomment-828726791_

To Reproduce

1. Setup a Netlify Form as described here and more specifically here:
   • Create a Netlify “form detection helper”. Something like:

     <!-- "/static/netlify-form-helper.html" -->
     <form name="test" netlify netlify-honeypot="bot-field" hidden>
         <input type="text" name="name" />
         <input type="email" name="email" />
         <textarea name="message"></textarea>
     </form>
     

   • Create the Svelte form component:

     <!-- "/src/routes/test-form.svelte" -->
     <form name="test" method="post" action="/success">
         <input type="hidden" name="form-name" value="test" />
         <input type="text" name="bot-field" />
         <p>
             <label>Your Name: <input type="text" name="name" /></label>
         </p>
         <p>
             <label>Your Email: <input type="email" name="email" /></label>
         </p>
         <p>
             <label>Message: <textarea name="message" /></label>
         </p>
         <p>
             <button type="submit">Send</button>
         </p>
     </form>
     

   • Ensure Netlify’s form processing is enabled.
2. If everything is setup correctly, Netlify should tell you that it detected a form named “test” for your site.
3. Now navigate to your https://example.tld/test-form and send a submission.
4. Even when taking care of all troubleshooting tips from here and here, neither verified nor spam submissions show up in Netlify’s form overview.

Expected behavior A form submission should be shown in Netlify’s form overview if submitted via my SvelteKit webapp.

Information about your SvelteKit Installation:

• @sveltejs/kit: next => 1.0.0-next.64
• @sveltejs/adapter-netlify => 1.0.0-next.4
• svelte: ^3.29.0 => 3.36.0
• vite: ^2.1.0 => 2.1.4

Severity Blocker, as Netlify’s simple form handling is one of the key benefits I’m using it for. However, right now I cannot use Netlify Forms with SvelteKit.

Additional context

• I tested all combinations of with/without honeypot field and success message.
• There is no problem when using Netlify Forms in a regular HTML file, e.g. via static adapter.
• I suspect that it might have something to do with https://github.com/sveltejs/kit/issues/930 ?!