R7800 cannot change device priority

I had this issue as well. I ended up doing a factory reset of the unit, then reconfiguring the settings the way I like all over again (really should never use a config backup file). I have not had the issue reappear again. The only thing I would also suggest is make sure you update the Dynamic QoS database, then reboot the router prior to configuring device QoS priorities the way you like.