I've given up and completely wiped the M1 Mac and will start over.

Thanks, Matt. What exactly are the "certificate chain assets"? As far as I know, there is only my certificate and the "Apple Worldwide Developer Relations Certification Authority" certificate. They are both in the "login" keychain. What is the "user-level Keychain"? Keychain Access shows two keychains under "Default Keychains": "login: and "iCloud", and two under "System keychains": "System" and "System Roots". This happens when I run codesign when using Terminal while logged in as my normal user account, which is an administrator account.