We made the PPC64 build_artifacts jobs not invoke deploy to experimental a long while ago.
We did this because we have a race condition between the build_artifacts jobs for a given commit for different platforms. This can and does cause various issues, but having the Fedora build invoke deploy-to-experimental and OST before the EL7 build is done is mostly harmless.
With PPC64 the issue is that when PPC finishes building before x86_64 we end up with 'noarch' packages in the repos that are newer then the equivalent x86_64. This in turn causes the experimental repos to be broken and OST to fail until the x86_64 build is done.
Fixing this will require creating some sort of a synchronization mechanism between the build artifacts jobs. This is not easy to do unfortunately.