Hi, Guix System reinstalls GRUB every time for convenience, but there's no need to run 'guix' just to fix GRUB.
You can reinstall it by hand if you know what you're doing, using GRUB's grub-install command. It doesn't even matter which distribution's GRUB you use, as long as it's not ancient. Guix's GRUB does not carry Guix-specific patches. What does matter very much is whether you use the UEFI or non-UEFI version. Can you share your operating-system's (bootloader ...) snippet? Form this distance, I can't tell you why one GRUB broke & the other one didn't. Kind regards, T G-R Sent on the go. Excuse or enjoy my brevity.
