I just upgraded from Plasma 5.8.5 to 5.9.0 in Arch Linux. I use kalu, which has a clickable icon in the system tray. However, since the Plasma upgrade, right-clicking this icon no longer works. (Left and middle clicking work as expected.) This is the only icon in the tray that doesn't work. However, I can simulate kalu's activity triggered by the right-click, so I suspect that it's not something inherently wrong with kalu. The dev asked me to enquire if anything had changed with the status notifier items between these two Plasma versions.

I have same Issue but with other gtk apps (keepass, cryptkeeper, xchat). Plasma 5.9.0, KDE 5.30.0, Arch Linux

I just found this bug report.