Mainly affected is the instant messaging icon, which sometimes is not shown. A white space is shown instead. Clicking on it does nothing. Disabling and re-enabling the "miscellaneous" entry in the system tray settings normally fixes it until the next reboot. Reproducible: Always
I can confirm this on Arch Linux, plasma 5.4.2 as well. The only different is it's the Network icon that most often disappears on my machine. Other icons can disappear as well but not as much. I also notice that only one icon is missing at a time.
*** This bug has been marked as a duplicate of bug 352580 ***
I also see this with Skype and Amarok icons. Plasma 5.4.3