Nesting changes made in Prefab Mode are not saved and the pop up which asks to save the changes while switching back to the main scene is not appearing if Auto Save is disabled.

Steps to repro:
1. Open attached project
2. Open 'A' Prefab Asset
3. Drag 'B' under 'A' (root)
4. Drag 'C' under 'B'
5. Switch back to the main scene

Actual Result:
Nesting changes made in Prefab Mode are not saved as well as the "Prefab Has Been Modified" pop up doesn't appear if Auto Save is disabled.

Expected Result:
Nesting changes in Prefab Mode are saved.

Note: Refer attached video.

Reproducible on:
2019.1.0b3

Working Fine on:
2019.2.0a4, 2019.1.0b2, 2018.3.0f2

Environment:
Windows & Mac