You haven’t posted actual info on your setup, but nevertheless…
- Boot to Manjaro
- Install Grub to the drive Manjaro is installed and update-grub
- Set this drive in BIOS as 1st boot
- Don’t use other drives’ Grub for booting
You may do more things “playing” with other installations, but always update-grub from Manjaro after any relevant changes on other installations and always use Manjaro Grub for booting.
It’s another distro issue, so even if we can make a search an assesment on this, it is not fair for other distros support. They need to make a living too…