Thanks for your reply, Tbigby.
This is a bit embarrassing. We did a factory reset and now the ADAC configuration seems to be working as documented. What was happening before is that after I disabled DHCP-RELAY and issued a sho qos diag, I was still seeing the precedences being used by DHCP and enabling ADAC would fail.
Now, since the factory reset and reconfiguration, the disabling of DHCP-RELAY releases the precedence as it should. ADAC can now be enabled.
Not sure what in the previous config would have caused that behavior.
Thanks again.