RemoteArtifacts are not being saved properly
This is the result of a long discussion on the Katello forums: https://community.theforeman.org/t/katello-4-1-2-1-404-error-through-content-proxy-due-to-incorrect-location-href/24812/26?u=dralley
TL;DR if you sync a repository on-demand multiple times against different repos, only the first set of RemoteArtifacts is saved. If the layout of the repository changes or the repository disappears, all of these URLs get broken, and the RemoteArtifacts are thus broken even if they were supposed to have multiple different potential sources.
I've confirmed this by syncing a single repository, changing the layout of that repository, and resyncing. Only the original RemoteArtifact will exist and the new ones will not. A script is attached to demonstrate this (if you look in the DB afterwards)
This is an especially severe issue because "metadata mirroring" and standard syncs have entirely different layouts, so re-publishing a mirrored repository or mirroring it after having otherwise not been doing so results in broken repositories due to all of the URLs changing.