Live-USB shutdown must be assisted manually by holding down power button.

Live-USB Amd64 on real hardware ISO version: 20180403.1
Issues relating to plymouth boot splash is extremely dim.
During shutdown process of live session no prompt to remove USB drive.
The shutdown has to be stopped manually by pressing the power button.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: plymouth 0.9.3-1ubuntu6
ProcVersionSignature: Ubuntu 4.15.0-13.14-generic 4.15.10
Uname: Linux 4.15.0-13-generic x86_64
ApportVersion: 2.20.9-0ubuntu2
Architecture: amd64
BootLog: Error: [Errno 13] Permission denied: '/var/log/boot.log'
CasperVersion: 1.392
CurrentDesktop: ubuntu:GNOME
Date: Wed Apr 4 03:32:10 2018
DefaultPlymouth: /usr/share/plymouth/themes/ubuntu-logo/ubuntu-logo.plymouth
LiveMediaBuild: Ubuntu 18.04 LTS "Bionic Beaver" - Beta amd64 (20180403.1)
MachineType: LENOVO 4236AT9
ProcCmdLine: BOOT_IMAGE=/casper/vmlinuz.efi file=/cdrom/preseed/username.seed boot=casper quiet splash ---
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=/bin/bash
ProcFB: 0 inteldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/casper/vmlinuz.efi file=/cdrom/preseed/username.seed boot=casper quiet splash ---
SourcePackage: plymouth
TextPlymouth: /usr/share/plymouth/themes/ubuntu-text/ubuntu-text.plymouth
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 03/06/2018
dmi.bios.vendor: LENOVO
dmi.bios.version: 83ET80WW (1.50 )
dmi.board.asset.tag: Not Available
dmi.board.name: 4236AT9
dmi.board.vendor: LENOVO
dmi.board.version: Not Available
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: Not Available
dmi.modalias: dmi:bvnLENOVO:bvr83ET80WW(1.50):bd03/06/2018:svnLENOVO:pn4236AT9:pvrThinkPadT420:rvnLENOVO:rn4236AT9:rvrNotAvailable:cvnLENOVO:ct10:cvrNotAvailable:
dmi.product.family: ThinkPad T420
dmi.product.name: 4236AT9
dmi.product.version: ThinkPad T420
dmi.sys.vendor: LENOVO