Steps:
1. Add "GameObject -> UI -> Text" to the scene
2. Drag it to Project browser to make a prefab out of it
3. select instance in the scene
4. Add "Audio source" component
==>
The dialog appears (attached) and says, that this change can be made only from Prefab Mode

Expected: Audio source component added as override to the instance and can be applied to asset

Repro also in Version 2018.3.0a6 (33e5def6e55f), looks like it was introduced by a new prefab workflows