I have seen this a lot of times and this causes some problems. It is nice to have a shared efi partitions, but sometimes problems occur. I am not a specialist, but rather a “learning by doing” specialist like many others.
- Manjaro has a customized grub bootloader is not compatible with the debian/ubuntu one as far as i know. This causes problems for newbes, when they update grub through debian/ubuntu and maybe other Distros.
- Windows like to reformat or overwrite the efi partition at updates. At least on my experience. Therefore, to avoid a broken bootloader, it is nessecary to have separate efi partition.
Grub can find other efi partitions and most UEFI’s can handle more than one efi partition. So to close these issues, i would suggest to make calamares creating its own efi partition like windows does.
The default installation should include a new efi partition and it should be avoided to use the efi partition created by windows or other Distro’s.