Hilary,
I had added it at line 215 for my code, not sure if your file will look exactly the same, so here’s a few lines before and after.
<?php echo $prefix; ?><label>
<?php if($accessibility_label!=""){ ?><span class="screen-reader-text"><?php echo $accessibility_label; ?></span><?php } ?>
<select<?php echo $attibutes_html; ?>>
<optgroup disabled hidden></optgroup>
<?php
foreach($input_args['options'] as $option)
{
It looks like this solution will add it to all dropdowns (select) fields as well, but only once. Were you possibly adding it inside the foreach loop?
If not, maybe output the $input_args array. There’s probably an indicator, or at least the class name being added to the field, that would let you test and add the optgroup only for comboboxes.
I ended up just using comboboxes, so didn’t test beyond that. Happy to take another look if it will help.
.
