Just to have some closure: This isn't an actual bug (not in gnome-shell, the kernel or any other package), the strange behaviour was caused by my keyboard (Logitech MX 5500) with *also* another 'normal' PS/2 keyboard attached.
The MX5500 forces the numlock on in hardware, but only when using those keys, otherwise it forces numlock off. A 'regular' PS/2 keyboard also attached means there is conflicting hardware forcing the state of numlock. When these 2 keyboards are attached at the same time it is not possible to force numlock on through software, but its state can be controlled using the numlock key on the second keyboard. Disconnecting that PS/2 keyboard removes the conflict and the MX 5500 works again with numeric keys. Best regards, Steven
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