AverageFrames incorrectly averages across scene changes

[HolyWu]

import vapoursynth as vs
core = vs.core

black = core.std.BlankClip(format=vs.YUV420P8, length=1, color=[16, 128, 128])
white = core.std.BlankClip(format=vs.YUV420P8, length=1, color=[235, 128, 128])
clip = core.std.Splice([black, white] * 3)
clip = core.misc.SCDetect(clip)
#clip = core.text.FrameProps(clip)  # see if the props are actually set
clip = core.std.AverageFrames(clip, weights=[1] * 3, scenechange=True)

clip.set_output()

VS R57