Qos marking being lost on ISR4331

Hey guys, I have a case I am working with Cisco TAC on but just wanted to post it here as well in case anyone has come across this.

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Topology:

Cisco SX20 <-----> Cat 9300 stack <--- trunked port-channel----> ISR 4331

Cat 9300 is running 16.6.4

ISR 4331 is running 15.5(3)S7b

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Endpoint is marking video packets correctly as AF41, auto QoS is applied on the interface using "trust device cts" and "auto qos video cts".

Both physical interfaces on switch which are port-channeled to ISR are set for "auto qos trust dscp". All other parameters on physical interfaces match. Port-channel is up and all seems normal.

Packet capture taken outbound on both physical interfaces of the portchannel shows packets retaining their AF41 tag. Taking the packet capture from port-channel is not supported.

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At the same time, a capture was taken ingress on the port-channel sub-interface showing packets are now marked CS4 instead of AF41. Neither the physical interface nor the sub-interface have any sort of policy-map, it's a very simple config.

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15.5(3)S7b itself is not old but I know the 15.5 code train has a pretty bad reputation...........I can't find a documented bug though.

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Thoughts?