Proxy clips result in imprecise cuts.
When using proxy clips, some extra frames appear on the video after rendered.
On the video below I show two different kdenlive instances, one with the original project, the other with the rendered video of the same edit. You can see the rendered video has one frame from another scene of the clip that was cut in the project.
I tried to disable the proxy clips, and voilà, the frame showed up on the original project.