auxiliary streams' quality is bad after Vpp Composition

I did a test to stitching four same pictures into one picture by sample_vpp.exe, but I found the quality of auxiliarystream worse than mainstream, is that normal?

The four input pictures resolution are 1920*1080, out picture resolution is 1280*720

stream=camera_capture.yuv
width=1920
height=1080
fourcc=nv12
cropx=0
cropy=0
cropw=1920
croph=1080
dstx=0
dsty=0
dstw=640
dsth=360

stream=camera_capture.yuv
width=1920
height=1080
fourcc=nv12
cropx=0
cropy=0
cropw=1920
croph=1080
dstx=640
dsty=0
dstw=640
dsth=360

stream=camera_capture.yuv
width=1920
height=1080
fourcc=nv12
cropx=0
cropy=0
cropw=1920
croph=1080
dstx=0
dsty=360
dstw=640
dsth=360

stream=camera_capture.yuv
width=1920
height=1080
fourcc=nv12
cropx=0
cropy=0
cropw=1920
croph=1080
dstx=640
dsty=360
dstw=640
dsth=360