How to reproduce:
1. Open attached project "SubGraph.zip" and scene "New Scene"
2. In Project window, open "position_sinewave_subg" sub shader
3. In the "Shader Graph" window, remove the connection between "Transform" and "SubGraphOutputs" (the last connection to the right)
4. Press "Save Asset" button and close the "Shader Graph" window
5. In Project window, open "Test_Shader" shader
6. Observe "position_sinewave_subg" (sub) node

Expected result: the sub shader node is updated and the "position_sinewave_subg" preview is black
Actual result: the sub shader node is NOT updated and the "position_sinewave_subg" is shown in green/yellow/red colors

Additional notes:
After restarting Unity the "position_sinewave_subg" subshader in "Test_Shader" shader gets updated, only after "Save Asset" is pressed the proper result is seen (the proper result is that the Cube disappears due to "position_sinewave_subg" outputting 0,0,0,0)

Reproducible with: 2018.3.14f1, 2019.1.1f1, 2019.2.0a14, 2019.3.0a1
Could not test with: 2017.4 Shader Graph was not yet implemented