Yeah, the grub2-snapper-plugin wasn't installed. I'm an anti-tinkering Linux user, so the package is not preinstalled in the ISO media installation. I saw the bug in my physical machine and I installed MicroOS/Aeon several times on GNOME Boxes and with UEFI support enabled. Thank you very much for your help. Now I see the snapshot entries in GRUB menu, but I think that the ISO image needs to be monitored or patched.