Vino advertises TLS even when require-encryption is false

I have an old Xubuntu system which I administer remotely from a Windows machine using UltraVNC. This doesn't support encryption, but since it's on a local net this is acceptable, and it has worked fine for years.

I have just installed Xubuntu 20.04 on a new machine, and have installed Vino from the repository and configured it using gsettings to the same values as on my old system. The settings are as follows:

org.gnome.Vino prompt-enabled false
org.gnome.Vino require-encryption false
org.gnome.Vino use-alternative-port false
org.gnome.Vino disable-background false
org.gnome.Vino disable-xdamage false
org.gnome.Vino alternative-port uint16 5900
org.gnome.Vino icon-visibility 'client'
org.gnome.Vino authentication-methods ['vnc']
org.gnome.Vino network-interface ''
org.gnome.Vino notify-on-connect true
org.gnome.Vino mailto ''
org.gnome.Vino lock-screen-on-disconnect false
org.gnome.Vino use-upnp false
org.gnome.Vino vnc-password 'xxxxxxxxxx=='
org.gnome.Vino view-only false

When I start the server, this is what I see:

10/06/2020 20:25:40 WARNING: Width (1366) is not a multiple of 4. VncViewer has problems with that.
10/06/2020 20:25:40 Autoprobing TCP port in (all) network interface
10/06/2020 20:25:40 Listening IPv6://[::]:5900
10/06/2020 20:25:40 Listening IPv4://0.0.0.0:5900
10/06/2020 20:25:40 Autoprobing selected port 5900
10/06/2020 20:25:40 Advertising security type: 'TLS' (18)
10/06/2020 20:25:40 Re-binding socket to listen for VNC connections on TCP port 5900 in (all) interface
10/06/2020 20:25:40 Listening IPv6://[::]:5900
10/06/2020 20:25:40 Listening IPv4://0.0.0.0:5900
10/06/2020 20:25:40 Clearing securityTypes
10/06/2020 20:25:40 Advertising security type: 'TLS' (18)
10/06/2020 20:25:40 Clearing securityTypes
10/06/2020 20:25:40 Advertising security type: 'TLS' (18)
10/06/2020 20:25:40 Advertising authentication type: 'No Authentication' (1)
10/06/2020 20:25:40 Re-binding socket to listen for VNC connections on TCP port 5900 in (all) interface
10/06/2020 20:25:40 Listening IPv6://[::]:5900
10/06/2020 20:25:40 Listening IPv4://0.0.0.0:5900
10/06/2020 20:25:40 Clearing securityTypes
10/06/2020 20:25:40 Clearing authTypes
10/06/2020 20:25:40 Advertising security type: 'TLS' (18)
10/06/2020 20:25:40 Advertising authentication type: 'VNC Authentication' (2)
10/06/2020 20:28:21 [IPv4] Got connection from client xxx
10/06/2020 20:28:21 other clients:
10/06/2020 20:28:21 WARNING: New width (1366) is not a multiple of 4.
10/06/2020 20:28:21 Client Protocol Version 3.4
10/06/2020 20:28:21 Ignoring minor version mismatch
10/06/2020 20:28:21 rfbClientConnFailed("No security type suitable for RFB 3.3 supported")
10/06/2020 20:28:21 Client xxx gone

Note that it says "Advertising security type: 'TLS' (18)" even though I have set "org.gnome.Vino require-encryption" to false. And as a result, I can't connect from my Windows box.

Can anyone tell me what to do to fix this?