What is the reason for the black background of the system tray icon when using QSystemTrayIcon on Linux?

Question

linux

In a Qt project, I want to use the application icon in the system tray.
I use the following code:
trayIcon = new QSystemTrayIcon(this);
trayIcon->setIcon(QIcon(":/app-ind.png"));
trayIcon->setContextMenu(mainMenu);
trayIcon->show();
The image has a transparent background. At the same time, the icon is displayed normally in cinnamon, but in unity and lxpanel, the background is displayed in black. Googling doesn't give anything clear. The Qt version is 5.3.2. I tried resizing the image according to the recommendations - the result is the same.