This appears to be related to this: Katello 3.18.1 Content Migration to Pulp 3 fails with "No declared artifact" message
and this: Pulp 2 to Pulp 3 content migration in Red Hat Satellite 6 fails with error Katello::Errors::Pulp3Error: No declared artifact with relative path "images/boot.iso" for content "<DistributionTree - Red Hat Customer Portal