Transcoding to x265 causes frozen frames

[Yuri Gorokhov]

Summary of the bug:
Transcoding video to x265 results in a few frozen frames in the output video.

How to reproduce:
Download the input video: ​https://github.com/yurigorokhov/ffmpeg_bug/raw/main/265_bug_input_video.mov

For context the input video was generated as such:

% ffmpeg -f lavfi \
    -i testsrc=size=1280x720:duration=50 \
    -vf "drawtext=text='%{n}':fontsize=72:x=(w-text_w)/2:y=(h-text_h)/2:fontcolor=white" \
    -c:v libx264 -preset ultrafast \
    265_bug_input_video.mov

Then transcode to x265

% ffmpeg -i 265_bug_input_video.mov -c:v libx265 -tag:v hvc1 \
    265_bug_frozen_frames.mov

Open the transcoded video and step through it frame by frame. At some point you will see that a sequence of frames is repeated. Unfortunately it's in a different time every time, but seems to be in the first 15 seconds consistently.

Here is a sample output video: ​https://github.com/yurigorokhov/ffmpeg_bug/raw/main/265_bug_frozen_frames.mov

It is frozen in the frame range 245 -> 250