NewMesh should be marked dynamic.

spine-unity/Assets/spine-unity/Mesh Generation/SpineMesh.cs

@@ -4,8 +4,10 @@ using System.Collections;
 public static class SpineMesh {
 	internal const HideFlags MeshHideflags = HideFlags.DontSaveInBuild | HideFlags.DontSaveInEditor;
 
+	/// <summary>Factory method for creating a new mesh for use in Spine components. This can be called in field initializers.</summary>
 	public static Mesh NewMesh () {
 		var m = new Mesh();
+		m.MarkDynamic();
 		m.name = "Skeleton Mesh";
 		m.hideFlags = SpineMesh.MeshHideflags;
 		return m;