I have a 5,5 myself, but not installed arch yet (I will because of Nvidia 🤪btw) and I have the same problem. The solution is to boot with the stick unplugged and holding alt until you get into the boot choosing thingy. While on that screen, plug in the usb you want to boot from. It should magically appear. I have no clue why it doesn’t appear directly, it even reads from the USB (blinking lights in the stick), but it even works, if it is plugged in while initial boot, just plug out plug in and ding it appears.
Maybe not, but somebody inevitably is. I was never a fan of the arch mods banning necrobumping because if that’s the topic thread that comes up in search engine results then it makes sense for the answer to live with the question.