Look guys, everybody at Yamaha were sure they got the 2017 bike perfect because Vinales was so fast on it, early on. Rossi struggled and complained that the bike felt strangely understeering, but of course nobody paid attention. That is, until Jerez. They worried for the first time. Then, when the problem reiterated, they tried to solve it but got into confusion, with some help from Michelin's inconsistency and from the feedback offered by their two factory riders.
Yes, because the two riders they have now have very different styles. With Lorenzo and Rossi, Yamaha had parallel feedback that made sense. The two knew what they wanted -- basically the same kind of bike, that was a good common base adjustable to their personal preferences via setup changes. Fine. But now, they have Vinales who is a purely instinctive rider, point and shoot style. He can tell whether he feels fine with the bike or not, but not much more.
Rossi is a corner speed rider who has become more and more analytical as he grows old. He needs the bike in a certain precise way. So probably he gives even too much feedback. But what he says doesn't match with his teammate's feedback, so engineers have a problem. And when engineers compare the two riders' subjective feedback with hard data, they have three things that do not match. So what happens? They go their own way, and Yamaha is becoming more of an engineer's bike as a result, and less of a rider's bike.
The 2016 bike issue: Rossi says it felt fine, but it had the problem of wasting the rear tire before the end of a race. Remember? Now, Zarco is so fast on the 2016 bike because he is the best at managing the Michelin tires. (By the way, he is a French rider who rides for a French team. Vive la France!) Great also for Dorna's business in France.
Interesting notes: the winner of Valencia 2017 was slower than the winner of Valencia 2016 by 20 seconds. Rossi was slower than himself by 15 seconds Similar weather and track conditions. The two factory Yamahas used 2016 frames, courtesy of Tech3: Rossi said it felt better, but he was slower by 15 seconds compared to his 2016 race.
So the tires must have changed really a lot, and Yamaha are those suffering the most. With the lonely exception of Zarco. It will be interesting to see how this situation develops.