Also, this will probably be relevant once the numeric accuracy is solved:
[https://forum.corona-renderer.com/ HDRI/Environment sampling causing Shadow artefacts ](https://forum.corona-renderer.com/ HDRI/Environment sampling causing Shadow artefacts )
With very slight changes in sun position, the internal accuracy for Sun/Sky is too coarse to produce a stable intensity (it’s a HDR map internally which is downsized to 1000x500px if I’m not mistaken, which causes the pixel representing the sun to have slightly different intensities depending on sampling). Make sure to use the ‘Accurate sampling’ option shown later in the thread by Maru.