A quick play on ITA suggests that it is not enough for that fare to be valid for there to be availability in B class. There must also be availability in (presumably) Q class, and there isn't. But there is availability in N class. That seems to be why, when I price a trip with that as the outbound flight and a randomly picked flight back on 27 December, ITA uses NHN5T1B5 to price that part of the itinerary. If you switch off the ITA availability checker, you will indeed see NHN5T1B5 being used ot price the outbound half of the trip.

I haven't read all of the rules published on ITA, but dual inventory fares like this often don't make it clear which other classes need to be available. However, I think that in this case the above behaviour, taken together with the first letter of each fare basis, gives a reasonable basis for thinking that QHN5T1B5 requires availability in both B class and Q class, and NHN5T1B5 requires availability in both B class and N class.