stuck down arrow/cursor key?!?

Hi,

even with no USB keyboard connected there seems to be an issue as if the down arrow key is stuck. This issue only seems to be active during booting, e.g. GRUB boot entry selection or while selecting the boot device.
It dows not occur in BIOS or when OS (Windows / Linux) is booted.
If I connect an USB keyboard I can use the cursor keys and work my way to the correct entry... very strange issue and I'm wondering what I'm missing here.

Best
Matthias