Strange issue: display_description() results in end of item/page output

[strarsis]

When using a custom template in theme (fdm-templates/menu-section.php),
using $this->display_description() inside will result in the item/page output to end right there.
Also the rest of WordPress page is not rendered.
Using $this->print_items() on that page works fine, the items of that section are correctly rendered.
Currently I use the condition + code (which isn't much) from display_description directly in the template file instead calling that function to avoid the issue and output the section description.