If I use the F1 + WinLock option during POST, the scroll lock keeps blinking, but the keyboard then will work with UEFI and GRUB. In that case, the scroll lock light keeps blinking until it is toggled off once again, which apparently is expected behavior. F1 + WinLock is supposed to start the blinking, but it actually starts right when the PC is powered on, and the keypresses do not work until the OS is loaded.