I discovered, that the queues in RouterOS can be escaped again. The particular setup, where I'm seeing this is a VPLS circuit, bridged to a lan-port. Queuing is established in the manner, as they suggest, by enabling firewalling for bridging and create the queue on the bridge.
So I send them a mail on the 22nd (4 days ago) about that and raised a ticket. Answer today is, they can't reproduce the problem. Guess I've got to figure that one out myself.