Hi Pascal,
I think it is possible to have the stars detected well and still have the seeing have a negative effect on the readings for auto focus. Even if a star saturates in one second, if the seeing is bad that well detected star can still be moving around from the seeing during the auto focus exposure. I found with my setup at 2350 mm and f10 that 10 seconds at 3200 ISO with a DSLR is enough to even out any seeing fluctuations. Three seconds would not be long enough with my Edge 9.25 if the seeing was bad.
Dean