This might be attributable to a mismatch of EFI/BOOT/Bootx64.efi and EFI/Manjaro/grubx64.efi - despite the differing file names, they should nonetheless be binary identical.
It’s possible that install-grub performed from a chroot environment might solve that, by replacing both copies (and grub at the same time).
One would need to boot from the Manjaro Installer, and enter a chroot environment from there; the link in your following post should provide sufficient instruction.
Regards.