Wrong Japanese input setting after click "Try Ubuntu"

1. boot Bionic final(20180426) iso with legacy bios (I'm using VirtualBox)
2. no input on isolinux screen and wait for ubiquity
3. Select 日本語 and Ubuntuを試す(Try Ubuntu) button
4. GNOME desktop is up. 4 keyboard layouts added as follows.
  Japanese
  Japanese (PC-98)
  Japanese (ODAG 109A)
  Japanese (Dvorak)

Expected Results:

Following 2 layouts added:

  Japanese
  日本語 (Mozc)

And "日本語 (Mozc)" should be selected by default.
This is because "日本語 (Mozc)" supports to input both Latin characters and Japanese characters.
So most of Japanese users always use "日本語 (Mozc)" and no need to switch "Japanese" layout.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
CasperVersion: 1.394
CurrentDesktop: ubuntu:GNOME
Date: Thu Apr 26 20:53:36 2018
InstallCmdLine: file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd.lz quiet splash --- maybe-ubiquity
LiveMediaBuild: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=ja_JP.UTF-8
 SHELL=/bin/bash
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)