Made the partials explicit in their section (i.e. user)

templates/forms/partials/input-user-user_name.html.twig

@@ -0,0 +1,11 @@
+{% block input_user_user_name %}
+{% if 'user_name' not in form.fields.hidden %}
+<div class="form-group">
+    <label>{{translate('USERNAME')}}</label>
+    <div class="input-group">
+        <span class="input-group-addon"><i class="fas fa-edit fa-fw"></i></span>
+        <input type="text" class="form-control" name="user_name" autocomplete="off" value="{{user.user_name}}" placeholder="{{translate('USERNAME')}}" {% if 'user_name' in form.fields.disabled %}disabled{% endif %}>
+    </div>
+</div>
+{% endif %}
+{% endblock %}