This might be hard to reproduce, but if you

1. Jump back in camera path timeline (bottom timeline) (setup basic keyframes first).
2. Hit the "Play Camera Path from Cursor Position" button.

When the scene is done loading after a few seconds, the camera path starts playing. At the very beginning of the playback, the mobs are not properly positioned, but rapidly move towards their correct location. For the next few seconds, the scene converges towards its actual state. Some entities might be properly located, while others rush to position themselves. In general, there is chaos at the first few seconds, while the playback is ongoing. Not sure, but I think this can happen with blocks too. The heavier the scene (more keyframes), the longer this effect lasts.

Can the scene load properly before the playback starts, to display it exactly as it is, without the early chaos?

Now, irrelevant from the above, but traveling back in time is more expensive than traveling ahead, but can it be cached to potentially eliminate that loading phase when jumping back in the camera path timeline?

Replay Mod v2.4.1 (This was present in older versions too)
Minecraft 1.16.1
Fabric 0.6.1.45

Turns out, the chaos in the first few seconds gets rendered in the final video too, making that part, pretty much useless.