SystemTray Icon is not shown in Taskbar extension, If it is running under Citrix XenApp 1912 LTSR CU3 ENV. Other non Qt application does shows it in same ENV. It appears this issue occurs with users having dual monitors. If they unplug and re-plug any of the monitor system tray displays the icon.

This can be reproduced with Qt's system tray example.
https://doc.qt.io/qt-6/qtwidgets-desktop-systray-example.html