Created attachment 5455 
little bugfix for little bug

package: xfce4-panel-4.11.0
plugin: tasklist

Let's start from a default panel configuration, then let's try to select the following options in the properties of the Window Buttons plugin:

1) Window grouping: ALWAYS
2) Show button labels: UNCHECK

after that, the grouped icons (and only them) will be incorrectly rendered in their "mini" version (you can see that more easily if you have a bigger panel size). 

Attached a small patch to fix the problem. :-)