Rob1 A longer rest lets roasty flavours ‘offgas’. Brazils and Sumatra can be ashy and burned quite easily, probably doubly so for decaf.
So if I understand this right, if the roaster is not highly experienced and skilled at roasting decaf (which is not easy), then (especially with Brazils and Sumatras) they can cause burnt flavours which then have to be solved through: a) longer rest periods, b) lower water temperatures c) shorter brew ratios, with b) and c) resulting in less extraction.
Fyi, I this morning encountered similar burnt flavour issues with an Ethiopian/Brazil decaf blend from the same roaster. I suspect it was/is the Brazil that is burnt, as this Brazil was also in the Sumatra/Brazil blend. I was able to eliminate the burnt flavour by reducing the temperature to 91C and shortening the brew ratio just slightly, to 1.85 (14 in, 25.8 out). Body is ok, but strength and flavour is still insufficient. So I suspect that while we got rid of the burnt tastes, we also (by reducing extraction) also substantially reduced the other flavors that we wanted. Will try a slightly longer shot at 91C to see if I can get some flavour back.