How to reproduce:
1. Open the “UnityBugSplinesSceneDirty“ project
2. Open the “SampleScene”
3. Add “SampleScene2” to the Hierarchy of the “SampleScene”
4. In the Hierarchy, select “Spline” GameObject
5. In the Inspector, add new Spline element to the Spline Container
6. Observe the Hierarchy window

Expected result: Only the scene with the added Spline marked as dirty
Actual result: Both scenes are marked as dirty

Reproducible in: 2.4.0 (2021.3.44f1, 2022.3.47f1, 6000.0.20f1), 2.6.1 (2021.3.44f1, 2022.3.47f1, 6000.0.20f1)

Reproducible on: Windows 10
Not reproducible on: no other environment tested