This bug seems similar to https://bugzilla.redhat.com/show_bug.cgi?id=2372978.
Importing the GPG keys for an additional repo fails.
In my case it happens on a Fedora Kinoite 43 with the Google Chrome repo (https://dl.google.com/linux/chrome/rpm/stable/x86_64) and the repo for vscodium (https://paulcarroty.gitlab.io/vscodium-deb-rpm-repo/rpms/).

If you already had these packages installed before, you now cannot update your system anymore.

Reproducible: Always

Steps to Reproduce:
1. Run Fedora Kinoite 43
2. add https://dl.google.com/linux/chrome/rpm/stable/x86_64 (or https://paulcarroty.gitlab.io/vscodium-deb-rpm-repo/rpms/)
3. Install packages from these repos, e.g. rpm-ostree install google-chrome-stable (or rpm-ostree install codium)
Actual Results:
The command exits with:

...
Resolving dependencies... done
error: failed to add subkeys for /var/cache/rpm-ostree/repomd/google-chrome-43-x86_64/linux_signing_key.pub to rpmdb


Expected Results:
The installation should succeed.