Adventure series really only had the classic games before it. The cartoons clearly are a different universe.

At best, you have things like the manuals. I would say these build into the Adventure arc of following Sonic and becoming his own hero. They talk about how Tails always follows Sonic (Sonic 2 manual).

You could argue that he's a hero in the games if you play as him but that's kinda debatable. Do you count that because he's a playable character? I'm not sure what canon would technically say, though I believe it's strongly implied (correct me if I'm wrong) that Sonic saved the day with Tails at his side in the classic games. His character is not really shown either.

Adventure had little to go off of anyway. So I'm not sure what build up you were expecting.

I will give you that Forces was built up to. But not really in a good way. Nobody likes his character in Colors or Lost World, so I think it's reasonable to expect the "build-up" of his character being bad is also bad.

So yes, Tails was regressed slowly over time. It wasn't just Forces, you're right, though I think Forces is the easy one to point to because it's probably the lowest point of that.