The user switcher widget on Neon 5.11.2 doesn't update the list of user sessions when a new session is opened. Sessions currently open do appear in the list of available sessions. Removing and re-adding the widget does show all the sessions.
Can confirm this bug in: Distribution: KDE neon Developer Edition - Stable Branches Plasma: 5.11.3 Frameworks: 5.41.0 Qt: 5.9.1 Kernel 4.10.0-38-generic Type: 64-bit