Collect + Sliding Window not working properly

Collect is not working properly when operating over a sliding window, like:

package org.drools;

import java.util.List

global List timeResults;
global List lengthResults;

declare OrderEvent
@role( event )
@expires( 2m )
end

rule "collect with time window"
when
$list : List( empty == false ) from collect(
$o : OrderEvent() over window:time(30s) )
then
timeResults.add( $list.size() );
end

rule "collect with length window"
when
$list : List( empty == false ) from collect(
$o : OrderEvent() over window:length(3) )
then
lengthResults.add( $list.size() );
end