At its core, the phenomenon you are observing is very much related to an issue in the context of rendering quadrilaterals, which are (internally) subdivided into two triangles which are rendered separately. See an extensive discussion about it in this thread. Spoiler alert: so far I’m not aware of a solution.
To show, that we are indeed observing the same phenomenon, I took a screenshot from your codesandbox and overlayed the diagonal as a white line:
