Some tray icons don't show menu

Hi all,
this has been going on for quite some time, but I’ve never reported it because there has been a lengthy period when KDE developers were struggling to decide what to do with the support for the “old protocol” to show icons in the system tray. So I did not want to report a problem that could be related to that.

Time has passed, and now system tray icons generally behave well. However, I still am seeing icons that don’t show their menu. Usually they are GTK applications, but I don’t really know if they’re using the “old protocol” or the new one.

One such example is the deja-dup backup application. When it starts (in background) it shows an icon in the system tray. On my wife’s PC (with MATE DE) if I right click on the icon I get a popup menu. On my openSUSE (with KDE Plasma 5) I cannot get any menu, either by clicking with the right mouse button or by clicking with the left one.

Does anybody else is seeing this? Did anybody find a workaround?
Is this a known bug?

Thank you in advance.
Cris