This is an issue on ShaderForge's end. ShaderForge is only able to generate multi-pass lit shaders when using lighting.
This isn't specifically a Spine issue. You'll get the same problem if you use that shader on Unity Sprites.
It should work well if the shader could just receive vertex lights and light the texture that way.
And vertex lit shaders were supposed to be a planned Shader Forge feature but it was canceled.
I once contacted acegikmo (Shader Forge developer) about it and, unfortunately, he said the feature is not a priority.
As an alternative, I recommend ToddRivers' sprite shaders: Sprite Shaders for Unity
If you can piece together logic from any shader you generate with Shader Forge and ToddRivers' code, and still benefit from some of the convenience of Shader Forge.
You could also make it use 1 or 0 alpha for clipping by writing and reading to the depth buffer. Unfortunately, that'll ruin any semitransparency, additive effects or alpha blending of any sort. (you'll need adaptive MSAA to get the edges smooth).
Beyond that, it's all hand-coding, I'm afraid.