Searchform inside list item tags not working when hitting Enter

Hi,

I’m currently designing a site (it’s not online, yet, so I can’t link it, sorry) and I came across a rather weird issue concerning the WordPress Searchbox.

Scenario: I have a menu in the topbar of my site and it includes the searchform that I add via the ‘wp_nav_menu_items’ filter.

Here’s my filter function:

function add_search_box_to_menu( $items, $args ){
  if( $args->theme_location === 'top-bar-r' ){
    return $items . '<li class="drop-down-search-box">' . get_search_form(false) . '</li>';
  }
  return $items;
}
add_filter( 'wp_nav_menu_items', 'add_search_box_to_menu', 10, 2 );

And here’s my searchbox that is added:

<form role="search" method="get" id="searchform" action="<?php echo home_url( '/' ); ?>">
	<div class="row collapse">
		<?php do_action( 'foundationpress_searchform_top' ); ?>
		<div class="small-8 columns">
			<input type="text" value="" name="s" id="s" placeholder="<?php esc_attr_e( 'Search', 'foundationpress' ); ?>">
		</div>
		<?php do_action( 'foundationpress_searchform_before_search_button' ); ?>
		<div class="small-4 columns">
			<input type="submit" id="searchsubmit" value="?" class="prefix button fa">
		</div>
		<?php do_action( 'foundationpress_searchform_after_search_button' ); ?>
	</div>
</form>

As you can see, I’m using FoundationPress but that’s irrelevant at this point, I think.
The issue I’ve come across here is that the searchbox works perfectly fine when I type my search term and then click the search button.
However, it does not work when I type my search term and then hit the enter key.

But that’s not all. When changing the
<li> tags to, say <div> tags in the add_search_box_to_menu function, the search via the enter key does work.

I have no idea why, and for my layout and consistency purposes throught the menu items I’d like it to work with <li> tags.
Has anyone come across this issue, too? Or maybe someone just knows this stuff enough to give me some clues? Because I got none.

Thanks in advance!