I recently bought an old HP Elitebook 820 G1 with a Windows 10 installed with uefi and secure boot. Massaged the BIOS until I could use Easy2Boot from an USB stick to boot a Linux ISO.
I booted manjaro-kde-18.1.5-minimal-191229-linux54.iso into the live desktop and ran the installer. Installed using the entire disk for Manjaro and chose to install GRUB in the Master Boot Record. Boot after installation: no bootable disk found! After several attempts with changing settings in the BIOS and new installations, still no luck. Checked the ISO checksum, file was perfect.
Installed Debian 10.2.0, it booted and ran perfectly. So the problem was not the hardware!
Booted manjaro-kde-18.1.5-191229-linux54.iso into the live desktop and ran the installer. After installation it booted and ran perfecty. Oh sweet joy!
My conclusion: manjaro-kde-18.1.5-minimal-191229-linux54.iso does not install GRUB properly in the MBR.