In blender 2.9, a Shadow terminator offset parameter has been added under Object Properties \ Shading. It is set to 0 by default, increasing its value fixes the problem.
Thanks to this, it is not necessary anymore to increase poly count by adding subdivisions.
Demo and explanations in this video.
Something like that generates Sun light.
You can try to get rid of by Shadow - Cascade Shadow Map - Max Distance, but seems to be a tricky solution.
This effect is such visible when Lamp light is switched to Sun where Strenght of light is too high (1000). Lower value to 3 is ussually fine in most cases.
Later I noticed - light still penetrates the ...