Doppelgänger
Superancient Member
Getting less screentime after their big moment and being useful in action scenes =/= losing their personalities after evolving.
This is an academic distinction. The writers don't have enough time to show off Pokemon personality traits during most battles. If you look at the two Kalos League battles, Hawlucha got the limelight against both Sawyer and Alain, with his personality working into the outcome. Talonflame, Noivern and Goodra were however unceremoniously and rather quickly executed. In the battle against Paul, Gliscor, Pikachu and Infernape got the most focus, with the three of them having more personality than Staraptor, Buizel and Torterra.
The time constraint is why in the Sinnoh League, there was an episode dedicated to familiarizing the viewers with Ash's old Pokemon and their quirks. There simply wasn't enough time during the battle itself to spend on personality, and if time was devoted, it was to the expense of a battle robot.
Alola hasn't had to deal with the problem because of how slowly paced the battles are. We have the same gradual build-up as in prior regions, but the Pokemon have more free time to show off their personality, and even take time in battle to do it.