Unity replaces original scene when importing it after it was exported as a package and renamed in another project and exported back.
1. Create a scene in Project 1
2. Export it as a package (scene 1)
3. Import exported package in a new Project 2
4. Rename scene (-> scene 2) in new project and update it (so it'll differ from original)
5. Export new scene as another package (scene 2)
6. Open original (Project 1) project and import new package (scene 2) into it
Expected: Scene 2 is a new scene next to Scene 1
Actual: Scene 1 gets updated with new content (replaced by scene 2 content).
At least this is the flow I've assumed would work and found this (IMHO) not quite logical behaviour.
Not a regression against 4.3.4