If you use Manjaro's grub to boot and not the fallback option, it will be fine.
To check if intel-ucode is enabled at boot, use
dmesg | grep microcode
Output will show.. something like...
pop@Aug:$ dmesg | grep microcode
[ 0.000000] microcode: microcode updated early to revision 0x2f, date = 2019-02-17
[ 0.911159] microcode: sig=0x206a7, pf=0x2, revision=0x2f
[ 0.911781] microcode: Microcode Update Driver: v2.2.
In the early days of intel-ucode, some distros put it into its modules but it does not have "updated early" as shown above in its output above. I think all distros have this fixed now. There are some distro's who do not put in intel-ucode into the kernels, but Solus is not one of them. You can try this in Solus and you'll find Solus has intel-ucode.
To boot manjaro from other OS grub, modify the last initrd line from
initrd /boot/intel-ucode.img /boot/initramfs-4.20-x86_64.img
Use the same version as the kernel in the linux line above it and it will be fine. Note space between the 2 image files.