You have to repair GRUB. Manjaro’s GRUB must be the one installed last, and it needs to be in control.
Boot up from the Manjaro live medium, open up a terminal and and issue the following command… 
su -
Then issue the following command…
manjaro-chroot -a
Select your Manjaro root filesystem from the menu and issue the following commands…
pacman-mirrors -f 5 && pacman -Syyu
grub-install --recheck --no-rs-codes --modules="part_gpt part_msdos"
update-grub
sync && exit
Let the commands take all the time they need, even if it takes a long time. Then, when they’re finished, you should be safe to reboot.