参考Unity论坛的一篇帖子，有一个回复我认为是最准确的。在不修改模型的前提下，只能通过修改URP设置中的Normal bias来调整。
This is because normal bias. Vertices on the cube edges are not stitched and have different normals directions. So normal bias push polygons in different directions and it's introduce gaps.
https://forum.unity.com/threads/shadow-bias-no-value-is-right.69750/
附送图文教程
https://www.immersivelimit.com/tutorials/fix-low-poly-shadows-in-unity