Why does my virtual screen reconnect immediately after I disconnect it?

Check its association settings. An automatic connection rule can reconnect a virtual screen while an associated display is online, even after a command disconnects it.

Open Settings > Displays > [virtual screen] > Associated Displays > Association Settings…. To manage the connection manually, turn off Auto-connect virtual screen and Auto-connect virtual screen when all associated screens are offline. You can retain the display association while disabling these connection rules.

The first rule connects the virtual screen when any associated display is online and disconnects it when none are online. The second does the reverse. It connects the virtual screen when all associated displays are offline and disconnects it when a partner comes online. With an association and either rule active, the normal manual connection controls can be unavailable.

Disconnecting temporarily removes the virtual desktop while keeping its configuration. Discarding deletes the virtual screen, so it is not a substitute for disabling an automatic connection rule. See Associated Displays for the other association behaviors.