I have Asus RT-AC68U Router with version "1.28.0000 -2.9-131 K26ARM USB AIO-64K ". When I setup QOS rules for port 1935 to manage upload priority for RTMP traffic to YouTube it works fine over IPv4. If I enable IPv6 on my computer and then start the stream it degrades after 10 seconds until it stalls. In the QOS Details I can see that it shows the IPv6 address of the computer and the correct destination port of 1935. However, it doesn't seem to be giving bandwidth for this connection.
If I turn off QOS and enable IPv6 the stream doesn't have any issues.