I can concur with your findings.
I ran Q4OS up in Gnome-Boxes, via the usual method, using Gnome-Boxes default settings, of 2Gb ram and 21Gb virtual disk.
Then made x5 new folders in my documents folder. Reboot and the iso image disappeared from the display panel in Gnome-Boxes, odd.
Created a second new virtual box, opened the installer and through to partition schema, where it informed me there was no virtual disk available and did I want to install to my disk. Then closed my session.
I’ve never seen a Linux system with .exe style installer, before.