mforms searchableselector doesn't disable hidden <option>s

Setup

• Open admin/webservice/forms.php and edit line 117, the line that adds a searchableselector for requiredcapability, and add the multiple option as follows:

  $mform->addElement('searchableselector', 'requiredcapability',

  get_string('requiredcapability', 'webservice'), $capabilitychoices, ['multiple' => true]);

• Purge your cache.

In all browsers...

• Go to the add a new webservice screen (admin/webservice/service.php?id=0) and click "Show more..." to display the search select box.
• Type 'ca' into the search box and observe that items are being filtered.
• Export jQuery to the global scope by entering the following into your JS console:

  require(['jquery'], function(jquery) { window.jQuery = jquery; });

• Shift-select from mod/quiz:emailwarnoverdue to block/calendar_upcoming:addinstance, making a visual note of the number of visible options being selected.
• Check in your console that the collection of selected options is of the same size using jQuery:

  jQuery(':selected').length