The System Tray has a number of trivially reproducible bugs that gather duplicates and user complaints:

• Dropbox doesn't remember hidden state in the system tray: https://bugs.kde.org/show_bug.cgi?id=378910

• If you succeed in getting dropbox hidden in the system tray, it disappears entirely: https://bugs.kde.org/show_bug.cgi?id=361613

• All entries can disappear from the system tray after modifying an item's hidden state: https://bugs.kde.org/show_bug.cgi?id=390890

• System tray icons become un-clickable and un-collapsible after an entry's visibility is changed to hidden: https://bugs.kde.org/show_bug.cgi?id=393630

• Left clicking on some system tray icons results in no action: https://bugs.kde.org/show_bug.cgi?id=365308

We should consider fixing them for Plasma 5.15