If you do this, it will install systemd-boot instead of grub, and you’ll have to go into the UEFI boot menu to boot another OS.
I’ve since found that changing installEFIFallback to false in /etc/calamares/modules/bootloader.conf works too, and lets you install grub which lets you select which OS you want to boot from a grub menu.
What prompted me to try changing the installEFIFallback setting were these lines from calamares-installation-log.txt posted by @0x6A7232 :
[PYTHON JOB]: "UEFI Fallback: True"
[PYTHON JOB]: " .. installing 'bootx64.efi' fallback firmware"
06:53:36 [1]: virtual JobResult Calamares::Python::Job::exec()
ERROR: Error while running: FileExistsError: [Errno 17] File exists: '/tmp/calamares-root-3cw0t75q/boot/efi/EFI/BOOT/bootx64.efi'
The error only happens if I install another distro (for example Kubuntu) first, then install OpenMandriva alongside it. If I install OpenMandriva on its own, it installs fine without any workarounds needed.