Different trainer here (Kickr direct-drive) but yeah, if you set resistance = 0 in the TR app you should be spinning 85rpm at ridiculously low watts (maybe 20W or 30W).
The first time I did an FTP test in TR it was an 8-min test I think (Dec 2017). When the TR app switched from Erg to resistance there wasn’t enough to push against and I was silly spinning at 150 ish watts when I needed to be at 230 ish. It was t obvious in the TR mobile app where to change it, but I figured it out and did well on the second 8-min interval in resistance mode.