box.html in Gantt Task not displayed of a GroupTask

Setting the box.html is not displayed. See following in documentation

box.html (if not specified, complete percentage is used for tasks, empty string for groups and milestone)

Setting box.htmlLeft has the expected result.

I changed the CSS so the taskgroup looks like a normal task. In the HTML the div has not the expected html:

<div unselectable="on" class="gantt_default_event_inner"></div>

Corresponding to the documentation it should have the specified HTML