Comment # 14 on bug 995258 from Michal Hocko

(In reply to Rich Coe from comment #13)
> I didn't set up anything in systemd, cgroup or otherwise.

Well, systemd tends to do many things behind your back. So I would double check
the pid controller configuration for the cgroup your process belongs to.
/proc/<pid>/cgroup will list all the cgroups the given pid is attached to. If
there is a pid controller then check the respective cgroup and its max pid
count setup. We have seen issues like that in the past.