How to reproduce:
1. Create a project with default Editor settings (Asset serialization mode = mixed).
2. Create scene1 with a cube
3. Create scene 2 with a sphere
4. Drag scene 1 over scene2 in the project tab
5. Click merge into new
6. Click use semantic merger
7. Nothing happens (no merged scene), even after project refresh or reload. It should throw an Error to make sure that asset serialization is set to "force text" in editor settings (How it is on Win)