I'm not sure if it's the result of a bad update or something, but suddenly I find that all my alt codes are typing the wrong characters. For example, Alt+168 should type ¿, but instead I get ◘.
I've checked through the Settings app and I'm set to United States region and English (US) language and the default US keyboard layout. I checked chcp in Terminal and it says code page 437.
I tried the normal DISM and SFC troubleshooting steps to check for a corrupt file somewhere, but no fix so far.
I'm still not sure the cause, but after finding that DISM bit that showed two keyboard layouts installed, even though Windows says it's only the one, I tried to research a bit more. 0809:00000809 is the keyboard layout for en-GB. I tried installing English (United Kingdom) as a second system language, then uninstalled it, and after rebooting, it only shows the en-US layout and my Alt codes are working correctly now. Hopefully this fix sticks...
Best I can tell, some update must have installed the en-GB keyboard layout, but did so in a way that wasn't visible in the Settings app, and therefore couldn't be disabled or removed. However, installing the language in the Settings app seems to have synced things up again, allowing me to remove the unwanted language and keyboard layout.
The code page still keeps resetting to 437, but otherwise seems to be causing no problems with my alt codes. I wouldn't call this perfectly fixed, but it accomplished what I was aiming for, getting my alt codes back to normal.