Codesign reports "Private key missing" for my certs after a reinstall

Thanks to APFS fun, I've had to reinstall from a blank slate here. I re-downloaded my developer certs and added them to my keychain (and they show up there under Certificates), but codesign reports that the private key is missing.


Given that I have downloaded my certs from the developer.apple.com site, and there is no error reported when I add them to the keychain, what am I missing that could cause this error?