The change from 8-6000' sounds like the differential increased which results in cabin altitude reducing.
The selection of the Flight Altitude window is paramount, if you set this above 37000 it triggers this situation.
Similar a failure of the selection being read by the controller defaults to the maximum allowed differential, this is a safety precaution. The same precaution maintains full maximum differential throughout flight, thus not reducing in descent. The safety feature is there to enable the crew to complete the flight without the requirement to descend. It does require crew actions to reduce the differential manually before or after landing.