That sounds like fasttrack enabled Fasttracked connections are "fast" because they skip firewall, queues, etc... There are still some packets going through the slow way, but it is just small percentage. Due to that, you can't see whole "forward" traffic in your queue. Try to disable firewall rule which says "action=fasttrack". Then, you should see full traffic going through.
As you were talking only about L2 bridging, I didn't expect you have defined forward rules in your firewall. Please, make sure your changes will not interfere with some other function of the router. (Disabling "fasttrack" rule should not break anything but it may decrease throughput of your router. That is not fault/error. It is just consequence of requested function) (See Edit2)
Edit: Also, keep in mind please that you can't select which bridge will use IP firewall and which will not. Sometime it is better to use more devices - one for each purpose, instead of trying to fit everything into one device.
Edit2: I just tested it and realized that you cant possibly do queue on bridge directly because there is no way to determine which direction is upload/download. you need to apply it to one or more interfaces connected to it. I tested it right now and it works. FIY: when setting rates, download = ingress, upload = exgress. Therefore, if you configure it on interface facing towards your clients, upload queue will affect client's download (and vice versa)