I’ve dug the problem more and found out that when the installer is started from the boot menu, apt-cdrom-setup udeb is also added to the installer, however when installer is started from the live-environment, this udeb is omitted somehow. This causes installation media not to be scanned as a candidate apt repository after installation completes.
The bug may solve itself if somehow live-installer also loads apt-cdrom-setup udeb during startup. I personally didn’t find a way to do so yet.
