Missing results in custom search while using a nuxeo-dropdown-aggregation

While creating a new search using Designer, if we add an aggregate bound to a directory to the page provider, and scaffold the search form layout, we end up using a nuxeo-dropdown-aggregation bound to that specific field.

However, upon searching we don't get any results, unless we add the `multiple` attribute to the aggregation element.

We should investigate if this is an accurate behavior, or if something is missing in Designer's side.

Solution
Remove multiple property from the element.