After installing and using apt-cacher-ng for a bit the signature was no longer good for jammy-backport on pop-os.
Here is an example of the message from the apt update on pop-os:
An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://apt.pop-os.org/ubuntu jammy-backports InRelease: The following signatures were invalid: BADSIG 871920D1991BC93C Ubuntu Archive Automatic Signing Key (2018) [email protected]
To resolve on the apt-cacher-ng machine issue the command:
rm -rf /var/cache/apt-cacher-ng/apt.pop-os.org/ubuntu/dists/jammy-backports/now run apt update on the pop-os machine.