That's strange, I have used MIDI learn to set up PolyBrute profiles on many of the V Collection instruments and it works great. The problem with Arturia creating a default mapping is that with so many controls on the PB, you'd never be sure what was mapped to what. Should the Juno filter be mapped to the ladder cutoff, the steiner cutoff, or the master cutoff? Should the lfo controls be mapped to LFO1 because that's the most obvious, or to LFO2 because that one has a delay control? What about controls that have no obvious equivalent on the PB, such as lfo -> filter amount? At least doing it manually you can use the mappings that make the most sense to you.