I'm not sure if it's them spinning the supplier issue of if they just got cheaper, but read this great Electrek article and Tesla's statement.
Tesla updates Model 3 interior with new headliner material
“As we continue to increase production of Model 3 and produce more high quality cars for customers, all Model 3 vehicles are being made with the same premium textile headliner found in our flagship Model S and Model X vehicles, which has always been planned at this stage of production.”
That "which has always been planned" statement rubs me the wrong way.
It comes across like "Why would you think to question what we are doing now?... because we already debated it internally and made up our minds and are done thinking about this..." "You outsiders seem out of step and behind the times with what we already decided." "You have no say in the matter so just accept it and move on..." It just comes across as overly dismissive.
*But* Tesla is in the driver's seat here. It is a sellers market with so many backlog orders that they can afford to cause some cancellations and disappointment and still sell all the cars they can build for the foreseeable future. If they were fighting for every sale, I bet their tune would be different.