I am using a persistent Lubuntu liveCD image. Now I intended to disable the autologin and hide the original user (lubuntu) in the login screen. during a reboot it tries to create a new user lubuntu and my changes in the config become invalid.
After some searching in old tutorials I finally found where the behavior is configured. If anyone is interested.
In the Lubuntu 19.04 LiveCD image the interesting scripts responsible for creating the user and login are in : casper\filesystem.squashfs\usr\share\initramfs-tools\scripts\casper-bottom\ is saved.