compositor: Composed output frame contains noise (artifact?) for NV12 format

See this image

command:

gst-launch-1.0 videotestsrc ! video/x-raw,format=NV12,width=640,height=480 ! compositor sink_0::width=640 sink_0::height=480 background=black ! video/x-raw,width=640,height=640 ! autovideosink