Forking https://bugs.schedmd.com/show_bug.cgi?id=11673 since I realized there are 2 separate bugs.

Tested on the current master branch, commit a10619bf1d482e189fc3f0dceed5ef459b410667
Running Slurm in a single node test config, on Ubuntu 20.04 with kernel 5.4.0-73-generic.

$ cat /etc/slurm/job_container.conf
BasePath=/var/run/slurm
AutoBasePath=true

When starting a new job, it has access to a per-job tmpfs mounted in /dev/shm:
$ srun --pty bash
$ echo $(date) > /dev/shm/date ; cat /dev/shm/date
Thu 20 May 2021 04:11:15 PM PDT

And if you launch another parallel job step, it will have access to the same instance of the /dev/shm tmpfs:
$ srun --jobid=14 --overlap cat /dev/shm/date
Thu 20 May 2021 04:11:15 PM PDT

This mount namespace is saved in /run/slurm/${SLURM_JOBID}/.ns:
$ findmnt -R /run/slurm
TARGET              SOURCE                 FSTYPE OPTIONS
/run/slurm          tmpfs[/slurm]          tmpfs  rw,nosuid,nodev,noexec,relatime,size=32594660k,mode=755
└─/run/slurm/14/.ns nsfs[mnt:[4026532561]] nsfs   rw

But if you restart slurmd (normally) while the job is still running, a new mount namespace will be created (4026532561 vs 4026532562):
$ findmnt -R /run/slurm
TARGET              SOURCE                 FSTYPE OPTIONS
/run/slurm          tmpfs[/slurm]          tmpfs  rw,nosuid,nodev,noexec,relatime,size=32594660k,mode=755
└─/run/slurm/14/.ns nsfs[mnt:[4026532562]] nsfs   rw

New job steps will now join a different mount namespace, that is empty, so the job steps are not in sync anymore:
$ srun --jobid=14 --overlap cat /dev/shm/date
srun: error: ioctl: task 0: Exited with exit code 1
/bin/cat: /dev/shm/date: No such file or directory

Whereas the original interactive job step (srun --pty) can still access /dev/shm/date just fine:
$ cat /dev/shm/date
Thu 20 May 2021 04:11:15 PM PDT