How to reproduce:
1. Open the attached project "case_1135638-SpriteAtlasUpdateBug_1" and the scene "SampleScene"
2. Create a Sprite Atlas asset in the Project Window
3. Attach "MyTexture" to the "Objects for Packing" list of the created Sprite Atlas' in the Inspector
4. Click the "Pack Preview" button
5. Right-click "MyTexture" in the Project Window and select "Reimport" from the context menu
6. Click the "Pack Preview" button again in the Sprite Atlas' inspector
7. Observe the Sprite Atlas' preview image in the Inspector
8. Enter Play Mode and observe the sprite texture drawn in Game View

Expected result: the "Pack Preview" button updates the Sprite Atlas with the reimported texture. The texture used by the Sprite Renderer in the scene does not change when in Play Mode
Actual result: the Sprite Atlas is not updated after reimporting "MyTexture" or pressing "Pack Preview". The Sprite Renderer in the scene uses the version of MyTexture before the reimport when in Play Mode

Reproducible with: 2017.4.23f1, 2018.3.9f1, 2019.1.0b7, 2019.2.0a8

Note: when reimporting "MyTexture" it is changed by a custom AssetPostprocessor but the applied changes are not seen in the SpriteAtlas as mentioned