We are having the same issue.Followed exactly the same process as previous times and the "Certificate Signature Verification failed because the signature is invalid" pops up. We tried renewal and new certificates, with the same result.

For anyone else having this issue, I had to add the latest AppleWWDRCAG3 certificate to the certificate chain on the plist needed for APNS renew. This might clear things a bit for you https://developer.apple.com/support/expiration/ "The renewed certificate will be used to sign new software signing certificates issued after January 28, 2021"