multiseat (via loginctl) will not work unless Wayland is disabled

When a second seat is set up via loginctl --attach, the greeter will not come up on the second screen unless WaylandEnable=false is set in /etc/gdm3/custom.conf (and gdm restarted).

For completeness' sake, the udev rules set up by loginctl are:

TAG=="seat", ENV{ID_FOR_SEAT}=="drm-pci-0000_6e_00_0", ENV{ID_SEAT}="seat1"
TAG=="seat", ENV{ID_FOR_SEAT}=="input-pci-0000_6c_00_0-usb-0_11_1_0", ENV{ID_SEAT}="seat1"
TAG=="seat", ENV{ID_FOR_SEAT}=="sound-pci-0000_6e_00_1", ENV{ID_SEAT}="seat1"
TAG=="seat", ENV{ID_FOR_SEAT}=="usb-pci-0000_6e_00_3-usb-0_1", ENV{ID_SEAT}="seat1"

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: gdm3 42.0-1ubuntu7.22.04.3
Uname: Linux 6.4.12-060412-generic x86_64
ApportVersion: 2.20.11-0ubuntu82.5
Architecture: amd64
CasperMD5CheckResult: pass
Date: Mon Aug 28 23:54:57 2023
InstallationDate: Installed on 2023-08-25 (3 days ago)
InstallationMedia: Ubuntu 22.04.3 LTS "Jammy Jellyfish" - Release amd64 (20230807.2)
SourcePackage: gdm3
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.gdm3.custom.conf: 2023-08-28T23:54:05.623349