I noticed this a few times throughout the game, but I am at a point in the game where it is super obvious and fairly strange.
I just beat Norman in Hoenn, and I have a decently leveled up team (51, 40, 44, 30). He was fairly challenging and I essentially won by toxic stalling the slakings. Without this strategy I would have needed to go find a bulky fighting type and train it to 50+.
Then immediately after gaining the ability to surf (from Normans badge) I encounter trainers with level 20 pokemon and I can just walk through one shotting everything.
I do have membership, and I understand that trying to balance a streamlined gameplay between those with and without it might be challenging, but if it takes level 40's to defeat Norman, the pokemon on the other side of the river should be 35+. I mean the magma and aqua grunts over by Lavaridge had level 35 pokemon, and that was 2 gyms ago.
Otherwise, the 3 regions have been great!