When you use Grub in Manjaro, you may notice that it has a different setup than many other Linux operating systems. Mainly, you may note that the Manjaro Grub configuration makes use of the Grub resume feature. This resume feature makes it so that Grub will remember your boot selection each time you access it. https://www.addictivetips.com/ubuntu-linux-tips/disable-grub-resume-manjaro-linux/