If you change scenes for the battle, you need to enable the Remember Combatants option (top of the Combatant Spawner settings). Otherwise returning to the scene will have a brand new combatant spawner that just spawns it's combatant.
The Scene ID is there to mark finished battles, so use that option if a battle should only occur once. That can be reset via schematics or automatically based on scene change count (set up in UI > Save Game Settings).