Win11 - Alt Codes suddenly typing the wrong characters

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.

Question by Ceetch on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.