Reproduced in SRP master of 4/12/19, 2019.3.0a1

New repro steps:

- Create graph
- Create scene color node
- Connect scene color node to albedo of unlit master output
Notice the output is correct.
- Create Add node
- Attach to input UV of scene color
Notice the output picks 1 pixel of the scene color target (still correct)
- Remove the Add node
- Convert Scene Color to Subgraph
Notice the output is still correct (because of your original fix)
- Now repeat the process of creating an Add node, but this time do it in the subgraph, and connect it to the input UV of the Scene Color node again
Notice the output has gone black (even though there is no black pixel on the scene color target) This is wrong.
- Now delete the Add node, which should revert the sub-graph to only outputting the default scene color
Notice the shader output is still black and at this point is unfixable. The sub-graph is busted.

Old Repro Steps:

- In the attached zip file, there is a shader and a sub-graph
- To reproduce, switch the connection in the graph between the Sceen Depth node, and the Subgraph that contains a Scene Depth Node
- Notice that when using the Subgraph node, the effect breaks