"Não é seguro retornar do nível de execução 1 para um nível de execução multiusuário".
WARNINGS
On a Debian system, entering runlevel 1 causes all processes to be
killed except for kernel threads and the script that does the killing
and other processes in its session. As a consequence of this, it isn't
safe to return from runlevel 1 to a multi-user runlevel: daemons that
were started in runlevel S and are needed for normal operation are no
longer running. The system should be rebooted.
link - embora este link possa ser substituído / removido em algum momento devido ao systemd, desculpe.