Captions aren’t exactly “frame accurate” anyway as each frame has just a very small piece
of information and only when a certain sequence is complete, it leads to some new letters
or line being ready for display.
In many use-cases, you want them to be frame-accurate. Final rendition to the viewer is only one use-case of FFmpeg (to be fair, the likely use-case of QSV) And you don't want errors accumulating across encode cycles.
Do you have an idea where/how I could find such stream?
You would need to record them off various television services as they use a wide range of encoder manufacturers. Ideally, you could also inject them into various encoders and force them to build complex reordering patterns and check they are frame accurate. Regards, Kieran