Ropieee - Cannot change screen brightness

I am running Ropiee ver 4.017 on a Rpi4 with the official Pi 7" display and a JustBoom DAC HAT.

All is running fine except one thing: the screen brightness slider is stuck at the midpoint of the range and does not respond to any attempts to move it (I want to be able to turn down the brightness for nighttime usage). The clock brightness slider does move and can turn the clock brightness up and down.

Any idea what I should be doing differently?