If you look at https://tracker.debian.org/pkg/jenkins it looks like that
jenkins is still in unstable with version 1.565.3-6 but in fact it has
been removed everywhere (the "package is gone" panel is a proof).
So the code updating the "versions" panel is not pruning the last entry
correctly when the package has been entirely removed from all known
repositories.
The relevant code is in distro_tracker/core/retrieve_data.py in the
UpdateVersionInformation object.
The execute function should really have some logic to drop the PackageData
entries for packages that have no associated source_package_versions.
Cheers,