Steps to reproduce:
1. Make sure there is only Assets folder in the Project folder
2. Open attached project
3. Open scene "boostrap"
4. In the top bar: My Asset Bundles > Build It
5. Play scene

Expected behavior: Red or green ball (depending on the script VariantLoader.cs) appears on the screen
Actual result: Asset Bundle Variant fails to resolve material

Reproduced with: 5.4.2p2, 5.5.0b11, 5.6.0a2
Not reproduced with: 5.4.2p1, 5.3.6p1 and older.

Note:
In the 13th line of script VariantLoader.cs user can change a name of a string "stuff.sd" to "stuff.hd". This way two different types of Asset Bundles are tested.
Reproduced on both Mac and Windows.