Additional information, the file that the installer is looking for is actually in a separate directory: /tmp/.../boot/efi/EFI/grub/grubx64.efi. I wonder why it’s not in the right place or why the installer is looking for it in the wrong place.
The EFI specification suggests that different distributions uses a subfolder of EFI to avoid collisions between when multibooting systems.
What is a bit puzzling is the file being looked for is in the EFI folder but the actual file being in EFI/grub - usually Manjaro creates a folder EFI/manjaro where the grub bootloader is stored.