XPS 13 9300 Page Up key combination stopped working, PgDn working fine

Hello,

all of a sudden my XPS' key combination FN+Up to trigger PgUp stopped working. As in: Doesn't register as a keystroke. Not in Linux, not in Windows, not in Bios. However, Page Down is working completely fine. Also some other key combinations aren't working anymore: I can't press Ctrl+Alt+Fn+F1 for example. If I disable Fn lock in BIOS pressing Ctrl+Alt+F1 works fine, however PgUp doesn't register as a key either. Edit: I just noticed Fn+F4 and Fn+F5 (which should trigger F4 and F5 respectively) aren't working either. 

So far I've tried every possible Fn lock setting, resetting BIOS settings to factory defaults, upgrading my BIOS to the latest version. Please keep in mind: this is also reproducible in BIOS, so this definitely is not an OS issue.

I'm completely out of ideas since this is happening on such a low level I don't really know if there's anything even left I could try - so any help would be appreciated