BBB Debian always needs a rebooto after flashing


when I flash BBBs with Debian (no matter if 8.3/ 8.4/ 8.6/ iot) is shuts down when finished. I unplug the power cord and the sd card. Plug the power cord back in. When it starts now, I cannot ssh to the device and my applications do not start properly. Rebooting it now everything works fine.
Why do I always have to reboot after flashing?!
Is this work as defined or a bug?

On first bootup after flashing the ssh key's are regenerated on
bootup, this can take up to 30 seconds.