Submitting forms go to the default page, and not a custom page

@G_G I see:

Upon submission the browser navigates to:

So the URL that it lands on is correct.

If I navigate directly to /form-submission/deleted I see:

Which is served with a 404 status

It just seems to be your custom 404 page, as it also appears for random other URL’s like:

/this/page/does/not/exist

So I again just fall back to what I said earlier:

Make sure it does.
Make sure the URL’s are correct.
Check what files exist in your deployment with the Deploy File Browser.