Hmm, you've got a decent point there. Well, there was the alternate US manual story that stated that Tails was just off vacationing solo after he's already known Sonic for a while. If anything, I'd place it after SA1, considering that was canonically the first time Tails ever felt accomplished for doing something on his own. But that's just getting into needless fan-revisionism I suppose. Story issues aside, Tails Adventure is a pretty neat game on its own. It has a really nice atmosphere and some great music.