QDockWidgets border distortion in Manjaro Kde

Please tell me how to solve this problem: when using QDockWidget in Manjaro KDE with version Qt5.15, the custom qss style sheet is not applied for the border of the pinned QDockWidget, as well as for the border of the QFrame placed in QDockWidget. Instead, the system decorator for the QDockWidget border is displayed, which cannot be disabled using the style sheet. Is there a way to disable the system decorator?

Normal borders (OS Windows 8, Qt 5.15):

Distorted borders (Manjaro KDE, Qt5.15)

I also contacted Qt technical support, but they don’t seem to know what this is about.