I think this is a Bug. I am not saying that I am using this functionality intentional, but it just happened. 🙂
One of our Graphic guys accidentally name a Texture "testsprite .png" (notice the white space)
This resulted in a Bug that the Texture could not be found in our Unity Project (using c# Runtime and 2DToolkit).
On Export to a .json File Spine Editor cuts off the Trailing White space. And therefore 2D Toolkit can't find the Texture "testsprite " because in the .json the Sprite is called "testsprite".
Best regards,
Robin