I'm having the exact same issue here (v1.21 firmware). It's extremely frustrating because it means I can only have one device on the bridged segment of the network accessible from the non-bridged segment, and to do this I actually had to enable MAC cloning and clone the MAC of the device I want exposed. If I uncheck cloning, I would expect that a device like this would pass all MAC addresses through untouched, but as you said, it doesn't it just replaces the source MACs of outbound traffic with it's own (which means no devices are accessible).
I think I'm going to have to ditch this device.