[FFmpeg-user] Issue with pix_fmt yuvj444p and libx265

[Samik Some]

I am facing a weird issue when encoding images to video in the yuvj444p 
format. The colors are off and there are artifacts which look like the 
image is enlarged and overlayed on itself. The issue persists if I 
extract the frames back from the video. I have providing links to 
samples below:

1) https://0x0.st/iybW.png - Original Image
2) https://0x0.st/iyb4.mkv - Image encoded using yuvj444p (1s video with 
2 identical frames)
3) https://0x0.st/iybJ.mkv - Image encoded using gbrp

The commands for encoding were:

ffmpeg -f image2 -framerate 1 -i %d.png -pix_fmt yuv444p -c:v libx265 
-crf 18 -an a.mkv
ffmpeg -f image2 -framerate 1 -i %d.png -pix_fmt gbrp -c:v libx265 -crf 
18 -an b.mkv

As can be seen encoding using yuvj444p causes a wash out and a overlay 
effect. Using gbrp works fine. yuv444p also works fine however it limits 
the color range.

What could be causing this issue?