How to reproduce:
1. Open tester-attached project (AssetBundleRepro), SampleScene
2. Enter Play Mode
3. Open the Profiler window and select the Memory module in Simple mode
4. Note the Textures/Meshes/Materials count/memory
5. Select the 'GameObject' in the Hierarchy window and in the Inspector window click 'Instantiate'
6. Note the increased Textures/Meshes/Materials count/memory
7. In the Inspector window click 'Clear' and 'UnloadUnusedAssets'
8. Note the Textures/Meshes/Materials count/memory
9. In the Inspector window click 'Unload Asset Bundle' and then 'UnloadUnusedAssets'
10. Note the Textures/Meshes/Materials count/memory

Expected result: the first call(step 7) to 'UnloadUnusedAssets' frees up memory
Actual result: only the second call(step 9) to 'UnloadUnusedAssets' frees up memory

Reproducible with: 2017.4.34f1, 2018.4.13f1, 2019.2.13f1, 2019.3.0b12, 2020.1.0a14