External BD SUP files with animations misses frames

Important Information

• mpv version: 0.36.0
• mpv origin: mpv.io nightly page / brew
• OS: macOs Monterey 12.0.1

Reproduction steps

mpv --sub-file=bd.sup clip.ts

where clip.ts is a standard 1080p23.976 video.

Expected behavior

The external .SUP file shows smooth animations, just like when it is muxed in a MKV container and played back with mpv 0.36.0.

Actual behavior

The external .SUP file skips Display Sets on play-back. Every two or three frames, the subtitle (overlay) is not updated and match the last frame while all frames should be different during an animation.
This does not happen when the bd.sup and clip.mp4 are muxed to mkv.

Sample file

https://0x0.st/HyKo.sup

Out-of-a-hat thought

At 23.976 fps, consecutives PG PTS should be separated by 3753.75 ticks @ 90 kHz. In practise, the PTS delta differs between Display Set by one tick regularly. If the rounding is performed incorrectly, mpv may not decode the right Display Set in the PGS file.

Reproduce the issue with mpv --sub-file=bd.sup clip.ts using the supplied https://0x0.st/HyKo.sup sample and a standard 1080p23.976 video. Compare external SUP playback with the equivalent subtitles muxed into an MKV, focusing on Display Set PTS differences and skipped animation frames. Done means external animated subtitles update smoothly like the muxed version.