For reasons that surely made sense at the time, the Huion H610Pro (V2) tablet can recognize when it’s connected to an Android device’s USB port and enter a special mode where the stylus only responds in a phone-shaped portrait rectangle over on the left side:
There’s a Vulcan Nerve Pinch button push to force the tablet into Android mode if it doesn’t automagically get there on its own, but AFAICT there’s no way to force it out of Android mode.
It’s a USB 2.0 device, but I had plugged it into a USB 3.0 port on my desktop box, whereupon it would enter Android mode on pretty nearly every boot. The only way to coerce it back into normal mode was to unplug it, replug it, then manually run the xsetwacom incantation to restrict the coordinates to the portrait monitor.
I just discovered it works perfectly when plugged into one of the few USB 2.0 ports on the box.
Apparently, USB 3.0 ports keep the thing powered all the time, whereupon it doesn’t see the proper sequence of events (or, perhaps, sees the Android sequence) during the next boot. USB 2.0 ports don’t do that and it works fine all the time.