The little window that shows the drag icon during drag and drops has an incorrect window type (at least on X11) as Tooltip
in its constructoore:
setFlags(Qt::ToolTip | Qt::FramelessWindowHint |
Qt::X11BypassWindowManagerHint | Qt::WindowTransparentForInput | Qt::WindowDoesNotAcceptFocus);

This may led window managers to mismanage it, as on X11 that window is supposed to have the type
_NET_WM_WINDOW_TYPE_DND