Qt disables the XSYNC protocol unconditionally

qxcbwindow.cpp

if (platformScreen->syncRequestSupported())
m_usingSyncProtocol = supportsSyncProtocol();

supportsSyncProtocol() returns "!window->supportsOpenGL()" and supportsOpenGL() is "type == OpenGLSurface || type == RasterGLSurface" ie. "true".

What was likely meant was to check whether "type != OpenGLSurface", no idea whether the supportsOpenGL() or the supportsSyncProtocol() query is wrong, but the result is https://bugs.kde.org/show_bug.cgi?id=356562 - supportsSyncProtocol() is false for every relevant surface.