I have a basic test scene with two meshlights and an area light, each has it's own light group (pink,green & yellow). Emission and diffuse direct AOV's are enabled with 'All Light Groups' selected.
On batch rendering a test frame the direct_diffuse gives me correct separation for each light group but the emission channels are all black. It does, however, make a new 'emission_default' channel with all the emissive meshlights in despite the fact that the 'default' light group doesn't exist in the scene.
Light path expressions for separating emission per light group also appears to broken (although I haven't tested this as thoroughly).
Is emission light group separation not possible?
MtoA 2.1.0.2
Arnold Core 5.0.2.3
Maya 2017 Update 3