One thing I don't like about Pokemon a lot and it's been like that since XY is that GameFreak doesn't put any work into building a coherent universe.

Let's take GTA for examples. It's divided into two canons, one is called 3D universe and it involves
>3
>Vice City
>San Andreas
>Some spin-off games

and the other is called HD universe, it involves
>4
>5

Both universes are completely separate, different entities. Pokemon doesn't have that at all, things just blend into each other even if there are mechanics introduced that completely change a lot (like mega evolutions) that are since ancient times. Sun/Moon for example introduces trainers from other games, some of them look literally the same, some of them completely different. This shouldn't be happening and XY should have been a restart to Pokemon, what GTA 4 was to GTA universe basically. Why is Kanto exactly the same in Let's Go remakes if mega evolutions exist? Hell, it should be different because technology is far more advanced than it was in FR/LG era, but that is another topic.

I feel like GameFreak makes shit up as they go along and there is nothing tangible to count on.

TLDR; GameFreak doesn't put any care or work into timelines and doesn't understand the concept of building coherent universes.