LoadScene.Single don't close the current scene

Hello everybody !
I 'm a beginner in unity, and i try to create a menu to switch between scenes.
But when i switch, i can see in the background the last scene.

The background camera is a solid color in all scenes.
I don’t understand why because i use LoadScene.Single to close all others scenes.
Before the switch :

After the switch :

Can anybody help me to find a solution please ?

Solution found : I had a singleton attached to each main gameObject. I write DontDestroyOnLoad on the function Awake on my singleton. So i remove the singleton on each gameobject, and i put a gameobject with my singleton.