New VNAV Descent
With a crossing ALT requirement and a long level segment to the next crossing requirement, the FMC (on all Boeings) behaves exactly as you describe.
Once the FMC crosses the first T/D it enters descent mode. It then blindly continues to calculate paths to the next altitude restraint, and then navigates them with thrust as required.
Try entering your new level (in your example F280) into the CRZ page. the FMC will return to cruise mode and recalcualte an idle descent to the next restraint.
Hope this helps