Accessing sub-array data from theme_field__custom_content_type($variables)
I think I get what you're doing...you won't be able to straight implode the items array if it's a sequential set that you're using to build up a list of key/value pairs...

There are probably a few different ways to do that loop, I'd try something like this:

$keys = $values = array();
$count = 1;
foreach ($variables['element']['#items'] as $item) {
  if ($count++ % 2 == 1) {
    $keys[] = $item['#title'];
  }
  else {
    $values[] = $item['#title'];
  }
}

$output = '';
for ($i = 0; $i &lt; count($keys); $i++) {
  $output .= $keys[$i] . ' - ' . $values[$i] . '&lt;br /&gt;';
}
$variables['card_subtype_list'] = $output;


Hopefully someone will propose a better solution, that's not exactly elegant. Apologies if I've misunderstood.