Finally, a solution...of sorts.
There is now a remedy circulating around for downgrading the MiBox 3 back to Android 8. I was able to do it successfully on my three MiBox 3 devices; they're working beautifully again with my HDHomeRun system. The solution is not without risks, however. It requires downloading some potentially dodgy software of uncertifiable origin and using it to replace the firmware on the MiBox 3. And there's a chance of turning the MiBox 3 into a brick.
The first step is to activate the developer mode on the MiBox 3; and in the developer mode, turning on USB debugging (Google it if you don't know how.) The second step is to follow the instructions on this site: https://en.xiaomitoday.it/problemi-con- ... -oreo.html
There are some YouTube videos that go through the steps, but I didn't find them so helpful. Bottom line: the process worked for me, but it may not be for the faint-of-heart since the downgrading process is a bit kludgey.