That is due to video compression. Read the last section of the getting started with scripting guide regarding how to re-encode the video into so called I-frames, i.e. into a video without compression.
https://discuss.eroscripts.com/t/how-to-get-started-with-scripting/2234
You can use FunExpander to simplify the process by getting a user interface to ffmpeg described in the guide above:
https://discuss.eroscripts.com/t/fun-expander-2-5-1-video-converter-with-ffmpeg-to-i-frames-added-updated-2021-02-19/4230