How to reproduce:
1. Open attached project (1221253.zip)
2. In the Project window select the "TestScript"
3. Observe the icon in the top left corner of the Inspector window
4. In the Project window expand the "TestLibrary" and select the "MyBehavior" script
5. Observe the icon in the top left corner of the Inspector window

Expected result: The "MyBehavior" script has the same icon as the "TestScript"
Actual result: The "MyBehaviour" script has a C# script icon from the old Unity Editor UI (before 2018.1.0a2)

Reproducible with: 2018.1.0a2, 2018.4.18f1, 2019.3.3f1, 2020.1.0a25, 2020.2.0a1
Not reproducible with: 2017.4.37f1, 2018.1.0a1

Notes:
1. The same issue occurs when the script is attached to a GameObject and its component's icon also uses the old Unity C# script icon. There is a GameObject named "DLL Object" in the scene with both the "TestScript" and "MyBehavior" scripts attached
2. Reproducible on a Windows 10 PC and Mac