How to restore disappeared Wayland session options under gdm3 on Ubuntu 20.04

Wayland sessions have disappeared from my gdm3 login screen. I typically use Wayland, but had switched temporarily to Xorg for a few days for a specific purpose. Now the Wayland options are gone.

Some possibly relevant observations:

• I know Wayland works still on my system: I can get a working Wayland session using LightDM or by starting a gnome session from a virtual console with XDG_SESSION_TYPE set to wayland
• WaylandEnable=false is commented out in /etc/gdm3/custom.conf (as it always was)
• I do have an Nvidia GPU, but it's disabled with prime-select intel (ie. the config is exactly as it has been for running Wayland).
• the relevant *.desktop files are still present in /usr/share/wayland-sessions/
• I haven't installed anything significant since the Wayland sessions were present a few days ago - just a couple of end-user apps.
• I have tried un- and re-installing the gdm3 package, and doing a `dpkg-reconfigure gdm3`, both to no effect

What might cause the session options to disappear and how can I reinstate them?