linux-module-cleanup.service deletes /usr/lib/modules entirely
Needless to say, this breaks the system. I'm not entirely sure why this happens, but disabling the service fixed the issue. This is on a fresh installation and happened consistently after several reinstalls. More information on the problem at the Manjaro forums.
Checking that $oldkern
isn't an empty string, just to on the safe side, is probably a good idea.