Steps to reproduce:
1. Upload a video, preferably a long one, and open the video editor.
2. Drag the timeline handle to choose a frame somewhere in the middle of the video (just so that there's some time to jump back to).
3. Press the play button.
4. While the video is playing, press the split button (the little scissors icon below the red timeline knob).
A new segment will be split off, but the video editor will jump to some point in the past. Sometimes just a few seconds, sometimes a minute. Seemingly randomly.
The video should just continue playing, without a jump in time.