Left control key nonfunctional in normal use

The Left Control key of my keyboard stopped working properly after the upgrade to bionic.

This is definitely a software problem, as I've been able to get it working by disabling and then re-enabling the 'Pointer location' switch in the Gnome Tweak Tool.

The behaviour is that when the button is pressed, it does not act like I would expect (for example, the Ctrl-Tab shortcut in web browsers stops working, or ^C in a terminal window), but instead causes the window to lose focus. (It's still on top, just not 'active')

I have no idea which package this should be filed against, and I've no idea what the pathway is for keyboard-related bugs.

Some similar problems seem to have been reported on some ASUS laptops - but I'm using a custom desktop PC and have never had issues before.

I am running Ubuntu 18.04 (bionic).

gnome-tweaks:
  Installed: 3.28.1-1
  Candidate: 3.28.1-1

mutter:
  Installed: 3.28.1-1ubuntu1
  Candidate: 3.28.1-1ubuntu1

gnome-shell:
  Installed: 3.28.1-0ubuntu2
  Candidate: 3.28.1-0ubuntu2

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gnome-tweaks 3.28.1-1
ProcVersionSignature: Ubuntu 4.15.0-22.24-generic 4.15.17
Uname: Linux 4.15.0-22-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.1
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Wed Jun 6 12:32:51 2018
ExecutablePath: /usr/bin/gnome-tweaks
InstallationDate: Installed on 2014-09-22 (1352 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
InterpreterPath: /usr/bin/python3.6
PackageArchitecture: all
Python3Details: /usr/bin/python3.6, Python 3.6.5, python3-minimal, 3.6.5-3
PythonDetails: /usr/bin/python2.7, Python 2.7.15rc1, python-minimal, 2.7.15~rc1-1
SourcePackage: gnome-tweaks
UpgradeStatus: Upgraded to bionic on 2018-05-10 (26 days ago)