Created attachment 119641 [details]
Konsole at left shows menu being opened at first time, at right menu has been reopened.

SUMMARY
on Wayland, with blur enabled, menus lose their translucency and become transparent. this is exhibited whether on application menus or on right-click (in the system tray for example).
this does not happen on X.
see attachment for an illustration of the problem.

STEPS TO REPRODUCE
1. Make sure Blur is enabled. Widget theme to use is Breeze.
2. Open a menu from a KDE application or right click on the desktop. Observe Blur works as expected.
3. Close the menu.
4. Reopen the same menu.

OBSERVED RESULT
Blur is lost and menu becomes transparent.

EXPECTED RESULT
Menu should look exactly the same as before (e.g. blur is still applied).

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Fedora 30 Beta (also observed in Fedora 29)
(available in About System)
KDE Plasma Version: 5.15.4
KDE Frameworks Version: 5.55.0
Qt Version: 5.12.1

ADDITIONAL INFORMATION
CPU: Intel Core i7-8550U
Graphics: Intel UHD Graphics 620 only