BTW, I did notice that the problem was gone during the time that systemd-oomd wasn't running, so that's definitely the cause. Unfortunately, the mask command alone isn't enough to prevent it from running, I'd have to either remove systemd-oomd-defaults or edit some config files. And this really should be fixed, it's just wrong that anything running in a DE is protected from being killed, while a non-DE login, or any command run from within that, isn't.