How to reproduce:
1. Create a new project
2. Add some tags to a list
3. Tag game objects with custom created tags
4. Save the scene and export package
5. Create new project
6. Import the package you have just exported
7. Open the scene and select game object you have tagged
8. Notice it still has custom created tag
9. Select Add Tags and notice that list is empty

Or it shouldn't completely export tags since it is project feature, or it should export it with whole list.