Finally experienced a flat on the trainer last night. I’m using a training tire and never thought to have a spare tube. (Training tire is thinner than Road tires, so can’t use those.) Failure seems to be a line on the “ground” side (outside) of the tube perpendicular to rolling direction of the wheel. (Sorry, no picture.). So not spokes. My guess is that I maybe introduced a small fold when installing. But if so, it held for months. Have not ridden outside since September.
Ordering some spares in case it is just the first sign of end-of-life. And will get some powder if it happens again.