I created an account just to share that I found out how to fix this! It was driving me crazy.
Fix: Make sure you're signed intoin to your Apple ID. The Monterey update automatically signed me out, and since the System PreferencesSystem Preferences > KeyboardKeyboard > Text shortcutsText shortcuts are synced up with your Apple ID/iCloud, it wipes them every time you restart the computer because it syncs back to nothing.
Good luck!