vaapih264dec does not output correct NV12/I420 frame for videos with size 640x480

gst-launch-1.0 filesrc location=/Videos/640x480.h264 ! h264parse ! vaapih264dec ! video/x-raw ! filesink location=/Videos/640x480.nv12

when decoding with above gst pipeline, the output 640x480 NV12 frame is incorrect, the output frame size is 471040, but not 460800(640x480x1.5)