I’m using Fedora 38 Plasma. I’ve been dealing with this for awhile. I’ve reported it to Tailscale but they state it would be an upstream (KDE) issue to be resolved. However, KDE has stated it is not their issue, but to file a Bugzilla.
Every time I attempt an update within Discover whenever there is a Tailscale update included, I receive this error:

    Bad GPG signature found:
    /var/cache/PackageKit/38/metadata/tailscale-stable-38-x86_64/packages/tailscale_1.46.1_x86_64.rpm could not be verified.
    /var/cache/PackageKit/38/metadata/tailscale-stable-38-x86_64/packages/tailscale_1.46.1_x86_64.rpm: digest: SIGNATURE: NOT OK

I can open a terminal and complete a dnf update with no issues and that has been my workaround for the past several months.

Reproducible: Always

Steps to Reproduce:
1. Open Discover to complete updates in Fedora Plasma
2. If Tailscale update is included, error will be seen
3. Complete updates in terminal using DNF instead of Discover. Or, complete package update of Tailscale within terminal and use Discover to finish other updates.
Actual Results:  
Error within Discover:

Bad GPG signature found:

/var/cache/PackageKit/38/metadata/tailscale-stable-38-x86_64/packages/tailscale_1.46.1_x86_64.rpm could not be verified. /var/cache/PackageKit/38/metadata/tailscale-stable-38-x86_64/packages/tailscale_1.46.1_x86_64.rpm: digest: SIGNATURE: NOT OK

And no updates at all can be completed.

Expected Results:  
Complete all updates, even Tailscale.