Early XY Ash was all about his interaction with other characters and showing off his personality, but it seems like people can only remember how he acted in XY&Z, which was where he indeed became a more serious person (and i found that justifiable given the nature of that arc). Regardless of that, they still focused a lot on his interactions with other characters. If anything, he interacted far more with characters like Sawyer than he ever did with Hau and he interacted more with Lysandre than he did with Guzma.
Also, i don't think it's fair to say he didn't have development in that series, when we had instances like Korrina's arc where he tried to learn how to dance so he could copy Tierno's style only to find out that he should just be himself, or Snowbelle's arc where he remembered why he became a trainer in the first place and it wasn't for the sake of winning battles, but becoming closer to Pokémon. We also had small developments in almost every gym battles, like him becoming a bit more patient and not trying to finish things as quickly as possible when he was up against Ramos.
There were also episodes where he tried to understand his pokémon better and was able to become closer to them, like when he couldn't understand why Fletchling was upset, then Ash realized his pokémon had a issue with his small size and encouraged the small bird to fight against Fletchinder. Or when he found out why Goodra was so scared of everything, helped him to get over that fear and then had to deal with the fact that Goodra belonged to the swamp, not with him. And that is just to name a few.