To reproduce:
1.Open provided example project
2.Open "Project/Assets/Scene.unity"
3.Notice the cube is blue
4.Open "Project/Assets/MyInclude.cginc" in a text editor
5.Notice the line "fixed4 col = fixed4(0,0,1,1); // blue"
6.Copy "Project/Assets/MyInclude.cginc" to "Project/MyInclude.cginc"
7.Switch back to "Project/Assets/MyInclude.cginc" in a text editor
8.Change line to "fixed4 col = fixed4(1,0,0,1); // red" and save
9.Reimport "Project/Assets/NewUnlitShader.shader"
10.Notice the cube stays blue
11.Delete "Project/MyInclude.cginc"
12.Reimport "Project/Assets/NewUnlitShader.shader"
13.Notice the cube turns red

Expected result: the cube turns red in step 10