This has unlikely anything to do with mock, but it could be another symptom of bug 1741935 or some other emulation bug. I can provide access to real s390 hw if needed.
It probably doesn't make sense to require additional info. This is
really unlikely to be a thing we could fix in mock. I'm switching
against qemu and marking as duplicate. Feel free to reopen if you
think it is worth tracking.
*** This bug has been marked as a duplicate of bug 1741935 ***