Removed references are sometimes kept in asset files and taken into account to determine AB dependencies.

Steps:
1. Open repro project in comment
2. Select the material in the AB1 folder
-> notice that the second slot is not assigned with any texture
-> so the assets in folder AB1 has nothing depends on AB2
3. On top menu -> Example -> Build Asset Bundles
4. Wait for a bit and check Abs's manifest file
-> you will see it says ab1 depends on ab2

Reproducible in : 2018.2.0b1, 2018.1.0f1, 2017.4.1f1