I confirm that CG’s suggestion works.
I just had the same problem:
I downgraded a 3040 client with the USB imaging Tool to version 8.6_013 then to 8.6_807 via the group firmware policy update. No problem at all there.
I then did the same for another 3040 client, but this time the 3040 after initial boot to 8.6_013 was then upgraded to 9.1.3112 !
So I reflashed to version 8.6_013 but did not reboot it to avoid a new unwanted upgraded and I checked the WMS (3.3.1) : the specific parameters for the client were showing the 9.1 options only.
I inferrred that the client had a specific configuration in 8.6 version to upgrade to the latest 9.1 version, so I guess that the unwanted upgrade was caused by this now ‘ghost’ 8.6 configuration.
So I followed CG’s advice and force unregistered the client and then delete it.
Upon registration, I got access to the 8.6 specific configuration.