Just spend 3 hours trying to figure out why systemd was killing my screen/tmux processes on logout. For anyone with the same issue:
1. In /etc/systemd/logind.conf set KillUserProcesses=no and KillExcludeUsers=root YOUR_USERNAME
2. Run sudo loginctl enable-linger YOUR_USERNAME
Was what fixed it for me in KDE Neon (Ubuntu 18.04).
piira.org is one server in the network