When building a RHEL ISO, "Kickstart is insufficient"

Description of problem:
Using Image Builder to generate an ISO leads to a "Kickstart is insufficient" message in the GUI.

It seems the installation_device directive (mentioned only in the Edge section of the doc) does not work.
The user has been added to the KS, locale (TZ, lang, KB) has been included directly in the tarball, but the FS customizations are not taken into account.

Version-Release number of selected component (if applicable):
osbuild-composer-46.3-1.el8_6.x86_64
osbuild-53-2.el8.noarch

How reproducible:
Always

Steps to Reproduce:
1. Use a blueprint with below customization settings
[customizations]
hostname = "golden-image-rhel86"
installation_device = "/dev/sda"

[[customizations.user]]
name = "postinstall"
description = "postinstall"
password = "redhat01"
groups = ["wheel"]
[customizations.timezone]
timezone = "Europe/Brussels"
[customizations.locale]
languages = ["en_US.UTF-8"]
keyboard = "be-latin1"
[customizations.firewall]
ports = ["22:tcp"]
[customizations.services]
enabled = ["sshd"]

[[customizations.filesystem]]
mountpoint = "/"
size = 8589934592

[[customizations.filesystem]]
mountpoint = "/var"
size = 5368709120

2. Generate an ISO

1. composer-cli compose start rhel86-iso image-installer

3. Install the ISO (s/sda/vda/ if running in qemu-kvm)

Actual results:
"Kickstart is insufficient" in the GUI (under the disk section).

osbuild.ks content:

liveimg --url file:///run/install/repo/liveimg.tar
user --name postinstall --password $6$GX1QdzFs13w5V22o$srZjjJSNsli38t1Fs92FOBqOvMCqCFtFPRGTk8aT4O73vcVIL8J2MtyCKfhLKKsEZjCtEMnnAbTEC5LAkgbzN0 --iscrypted --groups wheel

Expected results:
Take into account all the customizations or warn if it is not taken into account for a given image type.

Additional info:

• Upstream issue (which also reports network should be enabled on boot):
  https://github.com/osbuild/osbuild-composer/issues/2255