Problem: the sphere mesh that makes up the default preview in Shadergraph does not exist in any of the SRP packages or the scene. It has different UVs and is higher poly than the default sphere.
Repro steps:
1. Open a project containing Shadergraph
2. Drag New Shader Graph.shadergraph (attached) into your project and open it
3. Right click the preview and choose "sphere"
4. Save the shadergraph

Expected result: it should switch to the default Unity sphere but there should be some option in the right click menu to go back to the original preview sphere
Actual result: the mesh will be permanently changed once you change it and there will be no way to get back to the high poly sphere that the shadergraph uses by default, even if you choose "custom mesh"