Reported by Charxter:

Recently I've been using replaymod a lot more (1.15.2) and the first bit of time is always a still frame for (X) amount of time & then it jumpcuts to the actual replay.

Rendered video that shows the issue:
https://youtu.be/ezrKxHqKIMs

Log:
latest.log

Replay File with path preset:
2020_06_01_12_21_26_2.mcpr.zip

Sounds like they don't know how the bottom timeline works.
The timeline JSON in the replay doesn't appear to match the video but the first keyframe is at 13s, which fits with my suspicion.

Yes, indeed, that's the misunderstanding! Thanks for pointing that out.