On Kubuntu 18.04 an application (Reaper) suddenly lost its title bar. I have to right click on the application in the toolbar to kontrol kwin actions like "close window"
Curriously, child windows of the application (dialogs for instance) have their title bar and I can close them.
Other application display as usual.

The taskbar do not display correctly over the application

Here is a capture screen illustrating all theese behaviors:
https://www.webologix.com/media/public/Screenshot_20210102_135438.png

Any idea apreciated

Are you talking about the proprietary audio processing software? I doubt we have much influence over its integration into Plasma...