qt.scenegraph.time.renderloop log shows minus value of swap time

when

QT_LOGGING_RULES=qt.scenegraph.time.renderloop=true

is enabled and

QSG_RENDER_LOOP=threaded

with multiple window surfaces case,
sometimes log may give the minus value of swap time like:

qt.scenegraph.time.renderloop: Frame rendered with 'threaded' renderloop in 0ms, sync=0, render=2, swap=-2 - (on render thread)

and if

QSG_RENDER_LOOP=basic

always shows zero swap time.