After installing Leap 15.6 I did the first login in KDE and found many issues:
no sound, network manager not working, etc.
The reason was because the process systemd --user was not started.
Normally in the file /etc/pam.d/common-session-pc you will find this line:
  session optional        pam_systemd.so
which take care of starting systemd --user. In my installation such line is
missing.
When the rpm systemd is installed the post scriptlet runs:
  pam-config --add --systemd
which take care of adding the pam_systemd.so entry in
/etc/pam.d/common-session-pc.
Looking at /var/log/zypp/history I found the following:
# 2024-06-30 23:52:53 systemd-254.13-150600.4.5.1.x86_64.rpm installed ok
# Additional rpm output:
# ERROR: module /lib/security/pam_systemd.so is not installed.
# Running in chroot, ignoring command 'daemon-reexec'
# Creating group 'systemd-journal' with GID 469.
# 
2024-06-30
23:52:53|install|systemd|254.13-150600.4.5.1|x86_64||repo-sle-update|4b4498112e009db2bc50fd1739eaf226e74303f311af987a3488ef95f34cf05e|

When pam-config --add --systemd is executed, if the program does not find the
32bit module (which means if systemd-32bit is not installed before systemd) it
stops the execution with an error message and it does not add systemd support
in pam.

This behavior is also causing boo#1226123. In tumbleweed pam-config does not
throw an error if the 32-bit module is missing, only a warning. I think the
same should happen in Leap 15.6