Theme: Helium (v5.3.2 / g5_helium)
Gantry Version 5.3.2
Timber Version 1.1.2
The code from item.html.twig
{% extends 'forms/fields/select/selectize.html.twig' %}
{% block options %}
{{ parent() }}
{% if gantry.menu is not null %}
{% for group, items in gantry.menu.getGroupedItems() %}
{% if items|length %}
<optgroup label="{{ group|capitalize }}">
{% for key, item in items %}
<option
{# required attribute structures #}
{% if key == value %}selected="selected"{% endif %}
value="{{ key }}"
{# non-gloval attribute structures #}
{% if field.options.disabled in ['on', 'true', 1] %}disabled="disabled"{% endif %}
>{{ item.spacing|raw }}{{ item.label }}</option>
{% endfor %}
</optgroup>
{% endif %}
{% endfor %}
{% endif %}
{% endblock %}
Note: I have been using Gantry 5 hydrogen theme and I did not experience this problem.