Hi Ciro :)
Using Unity v2018.3.6f1 and the current Unity Asset Store build of the Unity Playground, I find that some console errors are thrown after toggling between Playground On/Off a couple of times (first time is fine). As a consequence, the user is no longer able to toggle between these modes. The Console Log seems to indicate that the issue relates from syntax changes:

Assets/INTERNAL/Scripts/Editor/DefaultComponents/GameObjectInspector.cs(101,49): warning CS0618: UnityEditor.PrefabUtility.GetPrefabParent(UnityEngine.Object)' is obsolete: Use GetCorrespondingObjectFromSource.'

Assets/INTERNAL/Scripts/Editor/DefaultComponents/GameObjectInspector.cs(112,35): warning CS0618: UnityEditor.PrefabUtility.RevertPrefabInstance(UnityEngine.GameObject)' is obsolete: Use the overload that takes an InteractionMode parameter.'

Assets/INTERNAL/Scripts/Editor/DefaultComponents/GameObjectInspector.cs(115,91): warning CS0618: UnityEditor.PrefabUtility.GetPrefabParent(UnityEngine.Object)' is obsolete: Use GetCorrespondingObjectFromSource.'

Assets/INTERNAL/Scripts/Editor/DefaultComponents/GameObjectInspector.cs(115,157): warning CS0618: UnityEditor.ReplacePrefabOptions' is obsolete: This has turned into the more explicit APIs, SavePrefabAsset, SaveAsPrefabAsset, SaveAsPrefabAssetAndConnect'

Assets/INTERNAL/Scripts/Editor/DefaultComponents/GameObjectInspector.cs(115,35): warning CS0618: UnityEditor.PrefabUtility.ReplacePrefab(UnityEngine.GameObject, UnityEngine.Object, UnityEditor.ReplacePrefabOptions)' is obsolete: Use SaveAsPrefabAsset or SaveAsPrefabAssetAndConnect with a path instead.'

Cheers,
Jim