systemd by default is configured to killing all user processes upon log out, including background running tmux or screen. To get back old behaviour several things need to be fixed.
[Login]
KillUserProcesses=no
KillExcludeUsers=root,<myusername>
loginctl enable-lingerBased on stack exchange post
mkdir -p ${HOME}/.config/systemd/user
cat > ${HOME}/.config/systemd/user/tmux-server.service << EOF
[Unit]
Description=tmux server
[Service]
Type=forking
ExecStart=/usr/bin/tmux -f %h/.config/tmux/server.conf start-server
ExecStop=/usr/bin/tmux kill-server
[Install]
WantedBy=default.target
EOF
cat > /home/tom/.config/tmux/server.conf << EOF2
set -s exit-empty off
set -s exit-unattached off
source ~/.tmux.conf
EOF2
systemctl --user start tmux-server