Reproduction steps:

1. Open "MinimalRepro.zip" project
2. "Assets" -> "Create" -> "Scriptable Object"
3. Notice a New Scriptable Object is created with a blue circle for its icon
4. Create a New Project and close it
5. Copy "MinimalRepro\Packages\test" to the New Projects Package folder
6. Open the New Project
7. "Assets" -> "Create" -> "Scriptable Object"

Expected Result: The new Scriptable Object has the same blue circle for an icon as in the first project
Actual Result: The new Scriptable Object has the default icon

Reproduced with: 2018.3.1f1
Did not reproduce on: 2019.1.0a13, 2019.1.0a1, 2018.3.0b1, 2018.3.0a1, 2017.4.18f1 (Package folder wasn't implemented yet)

Note:
Doesn't reproduce if using an Assembly Definition to reference the Package