How to reproduce:
1. Open attached project "case_1132278-ScriptedImporterNestedPrefabProblem" and scene "SampleScene"
2. Select the "Content" prefab In the Project Window
3. Press "Open" in the "Content" prefab's Inspector (top right)
4. In the opened text file change the text "Before" to "After" and save it
5. Inspect the "Parent" GameObject's child "Content" in the Hierarchy Window

Expected result: the "Content" child GameObject in the Hierarchy has the field updated to "After" as does its prefab
Actual result: a manual reimport of the "Parent" prefab is required to see the changes of the "Content" child GameObject in the Hierarchy

Reproducible with: 2018.3.7f1, 2019.1.0b6, 2019.2.0a5
Not reproducible with: 2019.2.0a6, 2019.2.0a7

Could not test with: 2018.3.0a5 and older (no nested prefabs)

Note: if the updated prefab instance in the Hierarchy is not nested, as is the case with the "Content" GameObject which does not have a parent, it is updated without requiring a manual reimport of the prefab