The scale of the RectTransform (X, Y, Z) has to be uniform / same value. I also can't see if you are using a TextMeshProUGUI object or normal TextMeshPro object. The Surface versions of the shaders should only be used if you do need scene lighting to affect the text objects. When using the Canvas System in Screenspace overlay, the scene light will not be affecting the text object since they are in overlay and Unity actually doesn't pass some of the information to the shaders. So this would only work when using Screenspace camera or Worldspace.