Any USB-stick connected at startup must be removed and inserted in another set of ports to be detected by GRUB. I.e. if a USB-stick is directly inserted onto the motherboard while starting up, it must be moved to the back of the motherboard when in GRUB to be detected. The port occupied during startup is 'broken' for that GRUB session.
Temporary fix: Insert USB-stick when in GRUB, and not before.
ON ANOTHER NOTE D16 together with GRUB is picky with USB-sticks (not the case for Seabios or e.g. x200)
Known good stick: 0951:1666 Kingston Technology DataTraveler G4 (8GB)
Known bad stick: 054c:05ba Sony Corp. (16GB)