Wordpress - Order get_terms using a Custom Field

A much shorter solution, just add this before foreach:

usort($terms, function($a, $b) {
    return get_field('issue_date', $a) - get_field('issue_date', $b);
});

Rather than outputting your terms in that initial loop, I would use it instead to build a new array, with your issue_date as the key:

$my_new_array = array( );
foreach ( $terms as $term ) {
    $issue_date = get_field( 'issue_date', $term );
    $my_new_array[$issue_date] = $term->name;
}

You can then loop through this new array in order:

ksort( $my_new_array, SORT_NUMERIC );

foreach ( $my_new_array as $issue_date => $term_name ) {
   echo "<li>" . $term_name . " " . $issue_date . "</li>";
}

This is untested.

I used a similar method, but I wanted to store more values from the taxonomy than the name and custom field value I gave, so I ended up storing it as an object and creating an array much like what is actually returned when you use the function get_terms.

Get your terms:

$terms = get_terms('your-taxonomy');

Then create a new array, storing them by my custom field value, which happened to be numeric in this case:

$newterms = array(); 
foreach($terms as $term) {
    $order = get_field('order', $term); //THIS MY CUSTOM FIELD VALUE                
    $newterms[$order] = (object) array(
            'name' => $term->name,
            'slug' => $term->slug,
            'term_id' => $term->term_id
    );
}

Sort them numerically:

ksort( $newterms, SORT_NUMERIC );

Then use the foreach loop to get the values of each object:

foreach ( $newterms as $newterm ) {     
        echo '<a href="#' . $newterm->slug . '">' . $newterm->name . '</a>';
}

So, essentially I am rewriting the array to use the key of my custom order, but in this case I needed the slug, name, and ID of the term, so I stored it as an object, rather than the method above.

My end goal was I set up an Advanced Custom Field so when a taxonomy term was created, it could be given a numerical order from the user, and then I could loop through the terms based on their desired order.

Hope this helps someone!