Moving the position of a field collection within a form causes "Remove" button to only act on the last item.

Talk about an edge case...

I have no idea what's causing this.

For usability, I enclosed a field collection field in a collapsible fieldset using form alter.

Everything worked fine, or so I thought. Turns out, when you hit "remove", it always removes the last item, even if you hit "remove" on the first item.

Turns out is it not the fieldset at all, but the nested array.

To replicate, put this in a node_form_alter where "field_collection_field" is the name of your collection field with unlimited values:

  $form['custom_array'] = array(
    'field_collection_field' => $form['field_collection_field'],
  );
  unset($form['field_collection_field']);

Then:

1. Go to create a node.
2. Enter a value in the field collection.
3. Click "Add Another".
4. Enter another value in the field collection, different than the first.
5. Click "Remove" on the first item.
6. Watch the last item disappear.

I couldn't for the life of me figure out how to fix this, so I'm just going to work around it some how.