Shutdown of attached Raspberry Pi using ssh results in reboots

As indicated here https://www.petoi.camp/forum/hardware/next-steps-pogo-pins-for-attaching-raspberry-pi-and-grove-vision-ai-module i attached a Raspberry Pi to the Nyboard using the 2x5 socket (and pogo pins).

I can ssh to the Raspberry Pi and use serial commands to interact with the Bittle, cool.

But unfortunately I cannot shutdown the attached Rapberry Pi as usual using "sudo shutdown -h now". It always reboots, my guess as it is attached to the Nyboard and gets somehow triggered to boot right after the shutdown command.

How can i achieve a safe shutdown / power off?

Thanks.