Try a partition manager to set the USB stick partition table to MBR and format to Fat32. Then reload the BIOS, rename and try flashback again. That has fixed the issue for other people. Good luck.
Z690 Hero, BIOS 2305, ME Firmware 22.214.171.1246, 7000X Case, RM1000x PSU, i9 12900K, ASUS TUF OC 3090TI, 2 x 16GB Corsair RAM @ 5200MHz, Windows 11 Pro 22H2, Corsair H150i Elite AIO, 4x Corsair RGB fans, 3x M.2 NVME drives, 2x SATA SSDs, 2x SATA HDs.