{{ if grouped_date }} parsed as False in 1.8.4, True in 1.7.5?

I have the exact same code and exact same content on 2 Statamic installs on Localhost:

{{ entries:listing folder="infos" sort_by="date" sort_dir="desc" group_by_date="Y-m" }}
{{ if no_results }}

    {{# do nothing #}}

{{ else }}
{{ if grouped_date }}
    {{ if first }}
        <div class="well">
            <h4>Archives</h4>
    {{ endif }}             
            <ul class="list-unstyled">
                <li>{{ grouped_date }} :</li>   
{{ endif }} 
                                <li><a href="{{ url }}">{{ title }}</a></li>
    {{ if last }}
                        </ul>
        </div>    
    {{ endif }}
{{ endif }} 
{{ /entries:listing }}

Everything is OK in 1.7.5!

In 1.8.4, {{ if grouped_date }} is evaluated as False and the browser is only returning the results for

<li><a href="{{ url }}">{{ title }}</a></li>
    {{ if last }}
                        </ul>
        </div>

Is this a bug or is there a new way to manage {{ grouped_date }}?