It doesn't work on the RT-AC66U, either. I think I've seen a few bugs mentioning Apple AirPlay before (uses multicast, doesn't work due to this defect) that were marked as WONTFIX for some reason. Multicast packets are not routing across two RT-AC66Us in media bridge configuration here. Flashing Tomato onto the one configured as a bridge makes it work, but significantly limits throughput as Tomato doesn't support the hardware acceleration supported by ASUSWRT.
It's pretty weird. Multicast will actually work for a very brief period immediately after changing what mode the router is in, but then it consistently stops working within a couple of minutes.