I un-installed the current version of yast-firstboot (2.9.3-0.2) and updated the install to 2.9.4-0.1 as you requested. The results were the same. I created a small, simple script in /usr/share/firstboot/scripts, touched /etc/reconfig_system, then rebooted. Firstboot service did run, but the script did not run. /var/log/Yast2/firstboot.log file was created, however it was empty.
I also tested with a stock control.xml file with the same results.
The information transmitted is intended only for the person or entity to which it is addressed and may contain CONFIDENTIAL material. If you receive this material/information in error, please contact the sender and delete or destroy the material/information.