{#=== INIT ===========================================================================================================#}

{% if context.content.taxonomy[taxonomy.slug] is defined %}
    {% set tags = context.content.taxonomy[taxonomy.slug] %}
{% else %}
    {% set tags = [] %}
{% endif %}

{% set attributes = {
    taxonomy: {
        id:       'taxonomy-' ~ taxonomy.slug,
        name:     'taxonomy[' ~ taxonomy.slug ~ ']',
        multiple: true,
    }
} %}

{#=== FIELDSET =======================================================================================================#}

{% extends '_base/_fieldset.twig' %}

{% block fieldset_type 'tags' %}

{% set fieldset_conf = {
    slug:  taxonomy.slug,
}%}

{% block fieldset_label_text  taxonomy.name %}
{% block fieldset_label_class 'col-sm-3' %}

{% block fieldset_controls %}
    <div class="col-sm-9">
        <select{{ macro.attr(attributes.taxonomy) }}>
            {% for tag in tags %}
                <option value="{{ tag }}" selected>{{ tag }}</option>
            {% endfor %}
        </select>

        {% if taxonomy.tagcloud %}
            <div class="tagcloud"></div>
        {% endif %}
    </div>
{% endblock fieldset_controls %}

{#=== INIT ===========================================================================================================#}

{% if context.content.taxonomy[taxonomy.slug] is defined %}
    {% set tags = context.content.taxonomy[taxonomy.slug] %}
{% else %}
    {% set tags = [] %}
{% endif %}

{% set attributes = {
    taxonomy: {
        id:       'taxonomy-' ~ taxonomy.slug,
        name:     'taxonomy[' ~ taxonomy.slug ~ '][]',
        multiple: true,
    }
} %}

{#=== FIELDSET =======================================================================================================#}

{% extends '_base/_fieldset.twig' %}

{% block fieldset_type 'tags' %}

{% set fieldset_conf = {
    slug:  taxonomy.slug,
}%}

{% block fieldset_label_text  taxonomy.name %}
{% block fieldset_label_class 'col-sm-3' %}

{% block fieldset_controls %}
    <div class="col-sm-9">
        <select{{ macro.attr(attributes.taxonomy) }}>
            {% for tag in tags %}
                <option value="{{ tag }}" selected>{{ tag }}</option>
            {% endfor %}
        </select>

        {% if taxonomy.tagcloud %}
            <div class="tagcloud"></div>
        {% endif %}
    </div>
{% endblock fieldset_controls %}

Fix tag select deliver an array
Fix tag select deliver an array