After updating a Puppet master server from Ubuntu 16.04 LTS to 18.04 LTS and re-enabling the Puppet package repository, Puppet related packages are not updated to their bionic version. The reason for this lies in the version numbers, which only differ in the distribution code name. But of course, "xenial" > "bionic", which means this would be a downgrade from apt-get's p.o.v.
Please use the distribution version number instead of the code name in package versions, for example, in case of puppetserver: 5.3.8-1bionic => 5.3.8-2ubuntu18.04 (we need the "2" here because "ubuntu" < "xenial", too). This way, this won't happen again in future releases.