Pettis was still taken down early by Chiesa, who was also trying to do all kinds of flashy stupidities, compromising position repeatedly. And Chiesa is not exactly the most athletic guy or best wrestler in the division.
Truth is, Pettis hasn't changed, and will never change. He is susceptible to being pressured and outworked by a pressure grappler.
It's not about primes or not. Him losing had to do with people figuring out his kicking heavy game and mediocre takedown defense is vulnerable. He and his fans like to believe in the myth that the only reason he started losing was because he wasn't fighting at the same level as before. That's just excuses.