Tried installing twice, once an automatic replacement of existing partition (Debian I wasn't using and had instaled to test) and the other with manual partition selection (selected / on sda3 and /boot/efi on sda1, exactly the same as the automatic version looked like.)
Install appears to work correctly, on initial reboot Grub does not pick up and has not been updated. I have to boot into Ubuntu and do a
sudo update-grub2 and then reboot and it tries to boot into Manjaro but I get the errors shown above (and on similar image here, not sure why subsequent boots seemed slightly different.)
This issue with Grub not getting updated I fear is a fault of Ubuntu, not yourself, it has been the same with every distro I have tried installing alongside, that I need to do a manual update afterwards.
Any idea why this wouldn't be booting?