I'm a gentoo user, so I don't know how either manjaro or arch lay out their partitions. But in my case, the grub.cfg (menu) file is actually on the root partition, in /boot. The efi partition contains only the grub loader, which I presume knows where to look for grub.cfg because that's how grub2-setup installed it.
It's possible you have a similar issue. Your manjaro partition may have a /boot dir which is where grub.cfg is located. Even if your arch partition also has /boot it won't matter if the grub loader doesn't read it.