gdm doesnt recognise smartcard when first inserted

We set the following in /etc/gdm3/greeter.dconf-defaults

[org/gnome/login-screen]
enable-smartcard-authentication=true
enable-fingerprint-authentication=false
enable-password-authentication=false

and restarted gdm.

GDM says "Please insert your SmartCard or enter your usernname". When the user inserts their smartcard to login nothing happens. We expect for SmartCard authentication to start but it does not.

If the cancel button is pressed then GDM looks at the card, starts the authentication process and prompts for the card PIN. The user logs in successfully.

We can teach our users to hit the cancel button but that seems like a bug. Other version of GDM (eg under RHEL) recognise when a SmartCard is inserted.

Description: Ubuntu Bionic Beaver (development branch)
Release: 18.04

gdm3:
  Installed: 3.27.92-0ubuntu1
  Candidate: 3.27.92-0ubuntu1
  Version table:
 *** 3.27.92-0ubuntu1 500
        500 http://us.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gdm3 3.27.92-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-10.11-generic 4.15.3
Uname: Linux 4.15.0-10-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.8-0ubuntu10
Architecture: amd64
Date: Thu Mar 15 15:21:47 2018
InstallationDate: Installed on 2018-03-14 (0 days ago)
InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180314)
SourcePackage: gdm3
UpgradeStatus: No upgrade log present (probably fresh install)