Command:
./ffmpeg -i ./img/%3d.png -pix_fmt yuv420p -vcodec libx264 -profile:v main -level 4.0 -bf 0 -bufsize 2000k -y ./raw.mp4
Here is my input and output
-- input: http://106.15.63.56/image/greenbackground.zip
-- output:http://106.15.63.56/image/green_output.mp4
I tried in Linux and macOS; the result is the same.
The color channels of the PNG contain the background color that you see in the MP4 since the video codec H.264 does not support alpha.
To set a background color of your choice, use
./ffmpeg -i ./img/%3d.png -vf "split=2[clr][bg];[bg]drawbox=c=black:t=fill[bg];[bg][clr]overlay" -pix_fmt yuv420p -c:v libx264 -profile:v main -level 4.0 -bf 0 -bufsize 2000k -y ./raw.mp4
Change the drawbox color to one you prefer.