Reproduction steps:
1. Open the attached project "ReproProj"
2. Open the “/Assets/Scenes/SampleScene.unity” Scene
3. Open TestBehavior.cs
4. Comment out “public Hex.FieldPoly<ITest> Field;“
5. Uncomment “public Hex.FieldPoly<int> Field;“
6. Save the file
7. In Editor, right-click the Project window and select refresh
8. In Hierarchy, find “Test“ GameObject
9. Drag “Test“ GameObject to the Project window
10. Right-click the Project window, and select “Show in Explorer“
11. Go to the Assets folder
12. Open Test.prefab in text editor
13. Observe the type of m_field

Expected result: “System.Int32“ exists
Actual result: “Hex.Test.ITest“ is present

Reproducible with: 6000.0.58f1, 6000.2.4f1, 6000.3.0b1

Testing environment: Windows 10 Enterprise 21H2
Not reproducible on: No other environment tested