Unlocking existing session needs two attempts after selecting Switch User

This has been occurring during the Xubuntu post-install test. It's not the same as Bug #1776475, as it's not necessary to have logged in as another user (or even for another user to exist) for the problem to happen.

My sequence of actions is:

*Create new user.
*Open a program (usually Thunar or Terminal).
*Lock session.
*Click on "Switch User". <-- This is all that's necessary to trigger it.
*Enter password to unlock my own session.
*Then there's a delay and screen goes black, then the unlock prompt appears again.
*The second time, my session unlocks.

This particular report is from a QEMU/KVM vm, but it's also been happening on a bare-metal install.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: lightdm 1.30.0-0ubuntu3.1
ProcVersionSignature: Ubuntu 5.4.0-26.30-generic 5.4.30
Uname: Linux 5.4.0-26-generic x86_64
ApportVersion: 2.20.11-0ubuntu27
Architecture: amd64
CasperMD5CheckResult: skip
Date: Wed Apr 22 15:30:29 2020
InstallationDate: Installed on 2020-04-22 (0 days ago)
InstallationMedia: Xubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200421)
ProcEnviron:
 LANGUAGE=en_NZ:en
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_NZ.UTF-8
 SHELL=/bin/bash
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)