Steps:
1. open any URP project (new URP template or UniversalRenderingExamples)
2. open a scene with a camera+assets (like, ToonOutlinePost.unity in UniversalRenderingExamples)
3. in hierarchy, select a camera
4. in inspector, right click on "Universal Additional Camera Data" > click "Reset"
5. close and reopen project

Expected: camera's original settings are not lost
Actual: camera's original settings are lost

Repro: 2021.2.0a9, 2020.3.0f1

Workarounds:
- If you do not have a backup and require the settings before "reset", do not close the editor window
- Use undo immediately after step 4 (CTRL-Z, CMD-Z or Edit > Undo)
- Create a preset from "UniversalCameraAdditionalData" so you have a backup of the state: https://docs.unity3d.com/Manual/Presets.html
- Otherwise, restore the scene (or prefab(s) with the camera) from a backup