Texture for disjoined mesh

Hi everyone,

I was trying to understand to mechanism when applying texture to mesh in Rhino. I experimented with two meshes with texture at different orientations ( both surface type mapping). Then I joined and weld them into one mesh. The texture is only distorted at the weld line. But I cannot edit the texture anymore from the mapping panel. How is the texture stored for the mesh under the hood? Is there any documentation I can refer to?

Best regards,