I tested this by flashing one of the prebuilt Tumbleweed 20210122 images for the Raspberry Pi 4 to a USB and tried booting it. I ended up with the same result where there was no USB after it got past the Grub 2 screen. I don't have a spare SD card at the moment since all the ones I have are being used. I might have to get more the next time they go on sale.