Gray screen after transcoding media pix_fmt yuv444p10le to yuv420p. Macroblocks visible

[Leonardo Diniz]

Summary of the bug:
When transcoding a video that uses h264 codec and pixel format yuv444p10le to h264 yuv420p on any ffmpeg version after 3.0.8 the transcoded media is all messed up with a gray screen.
I found out the using ffmpeg 3.0.8 the problem won't happen. However it hapens on version 3.0.9 and after.
How to reproduce:

Download the file from this URL:
​https://s3.amazonaws.com/player.hotmart.dev/test/ffmpeg_ticket/h264_pix_fmt_444.mkv
The file has 2.5MB
Open the terminal and move to the folder where you downloaded the file above.
Run the command:

docker run -t -v /$(pwd):/home leonardodinizhotmart/ffmpeg:3.4 ffmpeg -i /home/h264_pix_fmt_444.mkv -pix_fmt yuv420p /home/output.mp4

The output.mp4 file will have the problem.

If you run the command bellow the problem won't happen:

docker run -t -v /$(pwd):/home leonardodinizhotmart/ffmpeg:3.0.8 ffmpeg -i /home/h264_pix_fmt_444.mkv -pix_fmt yuv420p /home/output.mp4

The Docker file information can be found at ​https://hub.docker.com/r/leonardodinizhotmart/ffmpeg

The ffmpeg -v 9 -loglevel 99 log can be found at:
​https://s3.amazonaws.com/player.hotmart.dev/test/ffmpeg_ticket/log-3.0.8.txt
​https://s3.amazonaws.com/player.hotmart.dev/test/ffmpeg_ticket/log-3.4.txt

I see that ffmpeg reports a gray chroma error during the encoding on version 3.4. I can't see why it's doing this since that on version 3.0.8 it works just fine =(

[Leonardo Diniz]

The input file for the test

[Leonardo Diniz]

Log of the file using ffmpeg versoin 3.4

[Leonardo Diniz]

Log of the file using ffmpeg versoin 3.0.8

[Carl Eugen Hoyos]

Sound like a duplicate of #6717
If you know how to create such broken files, please explain!