pam_unix generates a lot of open/close sessions for my domain user

I setup a new VPS with ubuntu 18.04, including virtualmin/usermin. In auth.log I see a lot of

su[12936]: Successful su for domain by root
su[12936]: + ??? root:domain 
systemd-logind[148]: New session c315 of user domain .
su[12936]: pam_unix(su:session): session opened for user domain by (uid=0)
su[12936]: pam_unix(su:session): session closed for user domain 

in syslog, I see a lot of

systemd[1]: Started Session c314 of user domain.
systemd[1]: Started Session c315 of user domain.

domain is the user of my virtual server defined in the VPS. c314/c315 increased by 1 each time... It used to appear every 2-3 minutes, now it's every 5 minutes.

Reading on the internet about this, all the "solutions" were how to remove this logging from the log but nothing was explaining what are all those open/close sessions in the first place.

Also, when running loginctl list-sessions those sessions are accumulated in "active=yes" and "state=closing" mode and never disappear from the list. At the moment there are 95 such sessions.

What is happening on my VPS, who is opening/closing sessions so many times and why? Also, why those sessions never disappear from the sessions list?

Thanks

update

loginctl session-status c315
c315 - domain (1000)
           Since: Sat 2020-02-08 20:27:08 UTC; 23h ago
          Leader: 12936
             TTY: ???
          Remote: user root
         Service: su; type tty; class user
           State: closing
            Unit: session-c315.scope

Unit user-1000.slice (/user.slice/user-1000.slice):
└─session-2691929.scope
├─19035 sshd: domain [priv]
├─19051 sshd: domain@pts/0
├─19052 -bash
├─20124 sudo systemd-cgls -u user-1000.slice
├─20125 systemd-cgls -u user-1000.slice
└─20126 pager

Solution 1:

pam_unix sessions exit normally, as seen in the logs. Those increasing number of sessions are systemd-logind sessions, which for some reason remain open, even when they don't contain any processes.

A workaround you might try would be to force systemd-logind to kill all the session processes, when the session leader exits. You can do it by modifying the KillUserProcesses and KillOnlyUsers setting in /etc/systemd/logind.conf:

KillUserProcesses=yes
KillOnlyUsers=domain

and restarting systemd-logind:

systemctl restart systemd-logind

However, this does not answer the question, why the sessions are not closing by themselves, since the session scopes are empty.

Edit: About the difference between pam_unix and systemd-logind sessions:

• pam_unix sessions consist in a small record added to or removed from /var/run/utmp. You can list them with w or who,
• systemd-logind sessions are more heavy, as explained in the manpage of pam_systemd. Dangling systemd-logind consume much more resources. They are listed with loginctl list-sessions

Since you identified a possible culprit (in a comment to this answer), you can apply another workaround: replace

@include common-session

in /etc/pam.d/su with:

@include common-session-noninteractive

which does not contain pam_systemd. When you modify PAM files, the usual precautions apply: keep a root shell active (e.g. sudo -i) until you tested the new config, in case you break something.