50wt 3 ply thread is a thick thread. That extra ply makes a big difference. And probably is the same thread is in the bobbin so that is 6 ply in the stitches moving fast when fmqing. More then likely a universal needle is being used so a size 90/14 or larger is the best choice for 50wt 3 ply thread if the thread is linty with slubs.