Filing the bug as told by Richard, 
http://thread.gmane.org/gmane.comp.bug-tracking.roundup.user/9083

I've changed the classic template such that "topics" is a multi-select. On such a field, if one selects items, the browser will send the corresponding item numbers.

If one unselects all items, no number will be sent. This means that the last selected items stay selected.

Thomas