Not to discount anyone's experiences, but I feel like this comes up a lot with various ships. A couple of thoughts:
1) How the temperature feels can be relative. If an area is 75 degrees, and you just came in front sitting in the sun, it may feel downright cold. On the other hand, if you just came back from a nap in your interior cabin with the AC on full blast, it might feel rather muggy.
2) Heat outside, doors and windows can play a huge role in things as previously mentioned. If a line of people move through a doorway, and hot air blasts in, it's going to take time before that area cools. If this happens a lot in a high traffic area, they may struggle to cool it. While Horizon's sister ships are similar, they all have their own floorplan tweaks. There may be a door that's always opening on Horizon, that isn't on Vista.
3) If there is a problem, it may not be the AC. A failing window seal, or even an automated door that's getting stuck open or leaking excessive air can cause issues. A large window that doesn't properly reduce the sun's heat can also cause part of the ship to bake.
4) Where is the Horizon when this happens? She's been running a Southern Caribbean route that can be absolutely scorching in the summer. Little things or open doors may play a much bigger factor there then on ships further north.
Again, not trying to discount anyone's experiences here. But it's entirely possible that the AC is functioning perfect and other factors, possibly not fully in Carnival's control could be at play.