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

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

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