Comment # 5 on bug 1173106 from Manfred Hollstein

The issue is actually triggered by each kernel; their installation script
touches /boot/do_purge_kernels:

$ rpm -q kernel-default-5.3.18-lp152.19.2.x86_64 --scripts | grep purge
# Flag to trigger /etc/init.d/purge-kernels on next reboot (fate#312018)
touch /boot/do_purge_kernels
rm -f /boot/do_purge_kernels

After the next reboot, zypper purge-kernels then gets called and does its job.
However, it should *not* remove the devel and source RPMs!

Until this is fixed in libzypp/zypper, I'll use this recipe:

# systemctl disable purge-kernels.service
# zypper dup
# rm /boot/do_purge_kernels
# reboot