Yeh, from my experience internal wall frames are designed with minimum 20mm clearance, and
special brackets are supplied for connection to give the internal walls more lateral stability, whilst still enabling vertical settlement of the truss bottom chord via the slotted nailing points, which are supposed to be nailed at the top of the slots.
It would certainly cause some havoc with your cornices if there was 20mm movement though. What is the recommended time period between roof loading and plastering anyone?