This is a different use-case to OP and needs a different option to keep Windows as the default boot
option GRUB_SAVEDEFAULT should be disabled to stop any change to boot option being saved
The option GRUB_DEFAULT can be edited to use a specific number or name from the menuentry list on GRUB
Using the menuentry number is not good for the Windows boot option because the number will change if Manjaro kernels are added or removed from the list in GRUB
If OP wanted grub to boot Windows, option would be something like
GRUB_DEFAULT="Windows Boot Manager (on /dev/sda2/)"
This command will show Windows menuentry name(s) in GRUB specific to your system
sudo grep Windows /boot/grub/grub.cfg