Reproduction steps:
1. Create a new project "U1"
2. Add a new C# script "test" to "U1" project in "Assets\testcode" folder
3. Create another new project "U2" in the same directory as "U1" project
4. Open a command prompt as administrator
5. mklink /d "[project_directory]\U2\Assets\testcode\" "[project_directory]\U1\Assets\testcode\"
6. Open "U2" project and enter Play mode
7. Debug test.cs on Visual Studio

Expected: a breakpoint is reached
Actual: a breakpoint is never reached

Reproduced on: 2017.1.4p2; 2017.2.3p2; 2017.4.7f1; 2018.1.8f1; 2018.2.0f2; 2018.3.0a4