When submitting a form partial using a button that has name and value attributes set, this data is not included in the form data of the request. For example:

<form use-client-nav method="POST">
<button type="submit" name="x" value="y">Submit</button>
</form>

Form data is expected to include an x field, and will do when submitted normally, but not via a partial.

I think there's an easy fix, so I'll make a quick PR now.