The issue you're experiencing with sprite rotation angles not displaying correctly when using a grid in Spine is related to how Spine handles mesh attachments. When you convert a region attachment to a mesh attachment, the rotation of the region attachment is taken as the initial rotation for the mesh. However, once you start manipulating the mesh vertices, the rotation value becomes more of a reference rather than an absolute measure. This is because a mesh is essentially a collection of vertices, and when you rotate the mesh, you're actually translating these vertices in a circular path. The rotation value displayed in the editor is a reference to how much the vertices have been rotated as a whole, but it doesn't necessarily reflect the individual transformations applied to each vertex. This can lead to confusion when comparing the rotation values of different skins, especially after generating meshes.

To address this, you can use the "Freeze" option in the mesh properties to reset the mesh rotation to zero, which can help in maintaining consistency across different skins. Additionally, be aware that the rotation value is more of a guideline and may not accurately represent the visual rotation of the mesh if vertices have been manipulated individually.