By default, when a RealVNC Viewer user disconnects from RealVNC Server's Virtual Mode daemon their virtual desktop session is lost. When the RealVNC Viewer user reconnects using the same credentials, a new virtual desktop session is created; their previous session does not persist.
To change this so that virtual desktop sessions persist across disconnections, you need to enable the ConnectToExisting parameter in one of the Virtual Mode daemon's configuration files.
As an example, you can use the file /etc/vnc/config.d/vncserver-virtuald as shown in the below code snippet.
echo 'ConnectToExisting=1' | sudo tee -a /etc/vnc/config.d/vncserver-virtuald
sudo systemctl restart vncserver-virtuald
Virtual desktop sessions will always be lost when the computer running the Virtual Mode daemon is shutdown or rebooted.