1. Add Text to the scene
2. Drag Text GameObject to the Project browser to make a prefab
3. Double-click on Text prefab asset
4. Add "GameObject -> UI -> TextMeshPro-Text"
==>
GameObject is added to main scene

Expected: GameObject is added to PRefab mode scene

Note from devs:
This needs to be fixed by the TextmeshPro team:
In TMPro_CreateObjectMenu.cs they need to replace:
GameObject go = new GameObject(..)
with
GameObject go = ObjectFactory.CreateGameObject(..)
or create it with new GameObject() and add:
StageUtility.PlaceGameObjectInCurrentStage(..)