Programs which have option "hide when minimized" enabled and are isolated in separated desktop, will "disappear" when they are minimized, because there is no taskbar with tray area, no idea how to restore them
also as said previously if I open isolated Print queue window, there will be no tray icon, and if I minimized, it will also "disappear" with no tray icon, but this time I can restore it with alt+tab
I don't know if it's been already suggested but it is possible to make rehips tray icon change when it isolate programs so user will be aware that some programs are isolated