Since my initial report, either even more packages mysteriously vanished or I simply didn't notice the highly diverging number of packages in the ppc64 and ppc64le repositories.
However, as of right now, both repositories have only 24 packages in common.
274(!) packages are missing from the ppc64 repository but are available in the ppc64le repository.
Not all of these packages are actually unique packages. Instead, some are merely older versions of the same package (for instance, ansiable has four packages with three being outdated versions).
The trend, though, seems to be that any package that has more recent builds than Dec 07 is deleted from the ppc64 repository (including old versions), but the newer version never makes it into this repository.
My best guesses currently are that either deletion works, but pushing packages to this repository is broken for some reason, or that the package builds failed (which seems unlikely, unless your systems delete older packages automatically without waiting for a successful new build).
Something definitely looks odd there.
Mihai