@j8a thanks, it worked!
But since I did not need to follow all those steps, I just wanted to leave the differences on this thread so people who stumble upon it in the future know. In my case there is no need to chroot as my Manjaro install is booting normally, I just couldn’t use my grub install to boot another arch linux installation, so in my case all I needed to do was reinstall grub:
sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=manjaro
Before running sudo update-grub.
So the problem is sorted, but just for the sake of my linux learning journey could anyone confirm if it is recommended to reinstall grub everytime another OS is added to the disk?
My impression from the archwiki is that you just needed to update grub and let os-prober do its job, is this because of changes to grub made by manjaro?