You are correct. The problem is not the foundation, but what is called truss uplift. Some individual trusses, under special circumstances, buckle upward and raise the ceiling. These special circumstances include: high attic humidity, cold weather, part of the truss may be buried in insulation, or specific truss designs. These trusses are safe, but bothersome. You can hide the crack with molding installed 2.5 cm (1 in.) below the ceiling. If you change the drywall, attach the ceiling drywall to the partition wall without any nails on the truss 45 cm (18 in.) from the wall. This allows the ceiling to flex or bend with truss movement without cracking.