To reproduce:

1. Download attached project "TestVS.zip" and open in Unity
2. Open newly created .sln file in a text editor

Expected results: When it creates .sln file with more than one project it correctly assigns its unique name:

"Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Test_01", "Test_01.csproj", "{D7D7D5CF-6BE2-C3D6-DC90-A0743E764F5D}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Test_02", "Test_02.csproj", "{62BED521-7DBE-7C06-118D-934050B97E1B}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = ""Assembly-CSharp", "Assembly-CSharp.csproj", "{4A410388-96A7-2142-9852-BF06B7F1E716}"
EndProject
"

Actual results: When it creates .sln file with more than one project it incorrectly assigns its unique name:

"Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "TestVS", "Test_01.csproj", "{D7D7D5CF-6BE2-C3D6-DC90-A0743E764F5D}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "TestVS", "Test_02.csproj", "{62BED521-7DBE-7C06-118D-934050B97E1B}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "TestVS", "Assembly-CSharp.csproj", "{4A410388-96A7-2142-9852-BF06B7F1E716}"
EndProject"

Notes:
- This issue is reproduced on both Windows and OSX
- MS employees said that its a regression on 2018.1 because starting this version Unity is generating the solution files and not Unity VS Tools extension
- Forum thread: https://forum.unity.com/threads/issues-with-visual-studio-unity.514304/page-3#post-3482924

Reproduced on Unity 2018.1.0a1, 2018.1.2f1, 2018.2.0b5 and 2018.3.0a1
Not reproduced on Unity 2017.4.4f1
Regression on Unity 2018.1.0a1