The boolean modifier uses the brush object's materials when assigning materials to created faces, as opposed to using the material of adjacent faces on the original object.
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Closing this because AFAIK it's not a bug. Try adding materials with the same name to the boolean brush object. It cuts with those materials:)
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The example is contrived, so it's easy to work around, but a more complex brush shape wouldn't be as easy to recreate without the use of the boolean modifier.
Ah, I see now, though the way it works is slightly unintuitive, as materials are pulled from the boolean brush instead of the object with the modifier.
For a use case where the target object has the desired materials already applied, this makes the boolean modifier significantly less useful.
Example use case: Use several modifiers to generate geometry that then needs cutting.
The example is contrived, so it's easy to work around, but a more complex brush shape wouldn't be as easy to recreate without the use of the boolean modifier.
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It's not that easy because the mesh doesn't have volume data for material assignments.
When implementing something like this you need to make sure it works in all use cases and unfortunately here it's not possible. One solution is to split each material (in Edit Mode press P->By Material) into a different object and use the same boolean modifier on all of them. I know that one might argue there to be a check box "use material from adjacent faces" which might work in 99% of cases and I think that's a good idea but generally speaking it has tons of problems and low priority.
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Alright thanks for your help, I appreciate the timely response. Maybe one day this could be implemented (and like you said, a check box could be an effective solution, if implemented correctly), but I love the work the dev team does, especially on 2.8 and I agree that this is not a high priority feature.