Balance 20x, firmware 8.1
System -> Client List. When the mouse hovers over MAC address 06:EF:6F:71:43:80 the pop-up says “Private MAC Address”. Fine.
At AP -> Wireless client -> Wireless clients -> it says “Vendor Unknown” when the mouse hovers over the same MAC address
At Status -> Usage Reports -> Daily it also says “Vendor: unknown” when the mouse hovers over the same MAC address.
Finally, the User Manual says nothing about what a Private MAC Address is. This is a relatively new thing, as far as I know. At least for iOS and Android.