Tvheadend

I've installed V4.2.8-36 on a Raspberry Pi 3B+ (Raspbian Stretch) in the past with three DVB devices: one satellite and two terrestrial. And I was able to assign Config | DVB Inputs | TV Adaptors | (device name) | Priorities values (I used values of 100, 200 and 300) to the three devices so it used satellite (priority 300) as the first choice and the two terrestrial devices (priority 200 and 100) as second and third choice.

I'm now replicating the config on a Raspberry Pi 4 (Rasbian Buster). I've got everything else set up fine, but I can't make it remember or act upon the device priority. The value remains displayed as I've set it after I press Save, but when I look at another device which I have already set, its priority value is always zero, and if I start viewing a channel (Config | Channel / EPG | (press Play button beside a channel), it chooses the wrong device.

What silly mistake have I made which means that device priority values are not saved or acted upon?