Strange. I got it to work on a temporary profile I just set up. I just installed the app and granted it access here:
Settings > Accessibility > Screen Balance Service > Use Screen Balance Service (set to on)
I even messed with contrast and the settings were applied to everything outside of the app.
How did you install the app? I've read Android 13 blocks sideloaded apps from using accessibility services, but apparently apps from Google Play should be allowed? Maybe you can try uninstalling the app then installing it again from Google Play.