Retracting an event with an explicit expiration results in memory leak

package drools.tests
import drools.tests.MyEvent
import java.util.List
declare MyEvent @role(event) @expires(14d) end // <====== leaks
//declare MyEvent @role(event) end // <====== doesn't leak

rule "UpdateAndRetractOldEvents"
salience 10000
when
$old : MyEvent( $key : key )
MyEvent( key == $key, this after $old )
then
retract( $old );
System.err.println( "retracted $old" );
end

My test case is a loop that does this:

MyEvent firstA = new MyEvent( "first", "firstA" ); // key, value
MyEvent firstB = new MyEvent( "first", "firstB" ); // key, value
...
ksession.insert( firstA );
ksession.fireAllRules();
Thread.sleep( 50 );
ksession.insert( firstB );
ksession.fireAllRules();

Memory consumption stays flat if MyEvent is declared without an expiration time. Memory consumption increases linearly if MyEvent is declared with an expiration time.