No, F31 boots everytime under any condition. It's only for F32/33 afaict.I made some more tests. It's a race, 1 out of 10 tries succeeds and the chance that it does is improoved by inserting the usb drive while being in the bios. The F31 grub files i exchanged do not seem to have something to do with it.The race happens with the same probability regardless of GRUB Fedora release version?