This is a known problem. Apparently, it boils down to "Not all USB hard disks are completely compatible with the 'boot from USB' functionality of the Pi".
One of the other frequent posters here routinely reports that while he can boot from SSD from a cold power-on, rebooting (without power cycling) does not work.
My own recommendation is to skip the "Boot directly from USB" idea and just do it the old-fashioned way. Boot from SD card with the rootfs on USB. That always works. And as far as I'm concerned, is every bit as useful as the "pure boot from USB". Others will argue otherwise, but their arguments are not convincing.
"L'enfer, c'est les autres"
G fytc hsqr rum umpbq rm qyw rm rfc kmbq md rfgq dmpsk:
J lnacjrw njbruh-carppnanm vxm rb mnuncrwp vh yxbcb!