With systemd 230 we switched to a default in which user processes started as part of a login session are terminated when the session exists (KillUserProcesses=yes). See https://github.com/systemd/systemd/blob/master/NEWS#L3, the commit that made the change: systemd/systemd@97e5530cf20, and bug reports which gave the impetus for the change: https://bugs.freedesktop.org/show_bug.cgi?id=94508, systemd/systemd#2900.

Unfortunately this means starting tmux in the usual way is not effective, because it will be killed upon logout. There are a few option to avoid that, the best being:

systemd-run --scope --user tmux

This starts tmux as a scope unit under the systemd --user instance. It would be great if tmux could do this automatically. Probably the best way to do this would be to make the dbus call to org.freedesktop.systemd1.Manager.StartTransientUnit directly from tmux. See https://github.com/systemd/systemd/blob/master/src/run/run.c#L907 for how systemd-run does it, and https://www.freedesktop.org/wiki/Software/systemd/dbus/ for the description of the API.