How to reproduce:
1. Open attached "New Unity Project.zip" project and Scenes/SampleScene scene
2. Additively load Scene A and Scene B
3. Unload Scene A and Scene B (without removing)
4. Drag TestCube (1) Game Object from SampleScene to the unloaded Scene A
5. Observe the Hierarchy window

Expected result: Both unloaded Scene A and unloaded Scene B are present in the Hierarchy window
Actual result: Unloaded Scene B disappears

Reproducible with: 2018.4.29f1, 2019.4.16f1, 2020.1.16f1, 2020.2.0b14, 2021.1.0a8

Note: The following issues that appear after dragging an object into an additive scene that is not loaded:

1. The editor does not allow right-clicking on the additive scene (2019.1.0a10, 2019.4.16f1, 2020.1.16f1, 2020.2.0b14, 2021.1.0a8)
2. The label is not visible when renaming a Game Object in the Hierarchy window (2019.1.0a10, 2019.4.16f1, 2020.1.16f1, 2020.2.0b14, 2021.1.0a8)
3. It is impossible to reload the removed scene additively until the editor is restarted (2018.4.29f1, 2019.4.16f1, 2020.1.16f1, 2020.2.0b14, 2021.1.0a8)