@Dsinc I had 802.1p/DSCP QoS tested before without issue. But according to your description you may actually use Bandwidth Control? From what I know the bandwidth control on the switch only refer to the data link layer, not the Internet speed. If you want to limit the Internet download speed it's better to do bandwidth control via IP. Most of the routers can do that directly. More info about your whole network will be much appreciated. You can also contact their tp-link support if you find a firmware bug. 

@Somnus Yes, sorry got lack of clarity, I meant bandwidth control which resides under the QOS section

@Somnus I'm limiting the bandwidth at the port level, regardless of whether internet traffic or not. This is a hardware / firmware issue so knowing more about the network setup won't help! I will try contacting tp-link support. Thanks