initramfs resets usb 3 bus while booting from usb 3 drive

I boot ubuntu from a usb 3.0 thumb drive.
But during boot, booting stops at a busybox prompt.
Output of dmesg | tail says:
  22.888260 usb 4-2: reset SuperSpeed USB device number 2 using xhci-hcd

My work-around is to: unplug the usb 3.0 drive, plug it back in, then type exit.
After that, ubuntu boot normally.

Busybox version, stated at busybox prompt, is:
  v1.27.2

1) $ lsb_release -rd
Description: Ubuntu 18.04 LTS
Release: 18.04

2) $ dpkg-query -l busybox-initramfs
ii busybox-initra 1:1.27.2-2ub amd64 Standalone shell setup for initra

3) I expect ubuntu to boot from the usb 3.0 drive, without having to use my work-around described above. It used to work fine on older versions of ubuntu.

4) Instead booting stops at a busybox prompt as described above.

Here is my PC's model of USB 3.0 adapter:
$ sudo lspci | fgrep -i 'usb 3'
02:00.0 USB controller: Texas Instruments TUSB73x0 SuperSpeed USB 3.0 xHCI Host Controller (rev 02)

Thanks.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: busybox-initramfs 1:1.27.2-2ubuntu3
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
CurrentDesktop: ubuntu:GNOME
Date: Wed May 16 23:48:22 2018
Dependencies:
 gcc-8-base 8-20180414-1ubuntu2
 libc6 2.27-3ubuntu1
 libgcc1 1:8-20180414-1ubuntu2
InstallationDate: Installed on 2017-06-28 (322 days ago)
InstallationMedia: Ubuntu 17.04 "Zesty Zapus" - Release amd64 (20170412)
SourcePackage: busybox
UpgradeStatus: Upgraded to bionic on 2018-05-09 (6 days ago)