If I remember correctly, there was a discussion here from MichaIng and Joulinar to drop the dietpi user and let us create the username we fancy. Not sure if this is the case here.
Bug in the PREP script. Many thanks for reporting: https://github.com/MichaIng/DietPi/issues/4231
I’ll repack the images, luckily its nothing more to do than a mkdir -p /home/dietpi; cp -a /etc/skel/. /home/dietpi/; chown -R dietpi: /home/dietpi.