Wrong Keyboard Layout on Login after Sonoma Update on Dual Boot M1 - Can't Fix

I have a 2021 M1 Max Mac Book Pro with Monterey on one partition and Ventura on the other. Today I updated the Ventura partition to Sonoma. When I booted back to Monterey (which is on a totally different partition), I found I could not log in, even though I knew I was typing my password correctly. Finally, I noticed that the Keyboard "Input Source" had changed to "ABC - QWERTZ", which is the second of four possible keyboard input sources.

Once I noticed this, I was able to login by changing the keyboard layout to U.S.

But the thing is, changing to U.S. at the login in screen, or changing to U.S. after I've successfully logged in, has no effect. Every time I reboot, the login screen shows "ABC - QWERTZ".

Now, by deleting all input methods and leaving only U.S., the login screen did in fact show U.S. But if I add a second input source, no matter which language I choose, when I reboot, that is the input method selected at the login screen. (I tried these steps in Safe Mode as well, and the results were the same.)

What can I do to get the input source for the keyboard on the login screen to stick to U.S.?

(This problem pretty echoes the issue described in Mac Studio: wrong keyboard layout on logi… - Apple Community, for which the user found no viable solution. The difference in my case is that I after updating one partition, the other one seems to be permanently messed up.)

Thanks in Advance,

Mark