ISO images are not read properly by Rufus
adelie-live-kde-pmmx-1.0-rc2.iso
is read in the following way by Rufus 3.21.1949:
Scanning image...
ISO analysis:
Image is an ISO9660 image
Detected EFI bootloader(s) (from '/efi.img'):
● 'boot.efi'
● 'bootia32.efi'
Disk image analysis:
Image has an unknown Master Boot Record
Image is a bootable disk image
ISO label: 'Adelie-pmmx'
Size: 1.6 GB (Projected)
Uses: EFI
Using image: adelie-live-kde-pmmx-1.0-rc2.iso (1.6 GB)
This only allows UEFI boot on 32-bit systems, which is not very common; as far as I'm aware, only some Transformer tablets and MacBooks can even use 32-bit EFI. The same happens when the x86_64 ISO, but that is not as much of an issue as most 64-bit x86 systems can boot UEFI.