1. Create a new mod
2. Override the prefab {59121DD52FF62112}Prefabs/MP/Campaign/CampaignMPGameMode.et (Right Click -> Override in ..)
3. Open the World Editor and load the CTI_Campaign_Arland
4. Ensure the scenario uses the overriden prefab
5. Launch the game (F5) and ensure it did not give any script errors on start
6. Stop the game
7. Edit the overriden prefab. Add the SCR_ArsenalManagerComponent to it and save

8. Load the CTI_Campaign_Arland again. Ensure the prefab has updated and both the "ArmA" and "Mod" components are present in the prefab. Pay attention at the SCR_CompositionSlotManagerComponent

9. Launch the game. Notice the script error. Debug the error. Notice it is caused by the missing SCR_CompositionSlotManagerComponent component in the GameMode

10. Stop the game and inspect the GameMode in the World Editor. Ensure it has the SCR_CompositionSlotManagerComponent component.
11. Edit the prefab from step 2 and remove the added SCR_ArsenalManagerComponent. Save the prefab.
12. Load the CTI_Campaign_Arland again and start the game. Ensure there is no more script error.

Expected result:

Adding a component to an overriden Prefab should not erase the prefab own components, unles they are removed explicitly.