In an attempt to get Sonic into Knux's version of AIZ Act 2 (still can't do this), I somehow bypassed the miniboss sequence and got myself into Knux's location for the miniboss, but nothing appeared. Yet, by going to the right, I found myself at the entrance for Act 2, without finishing Act 1. Through Debug mode I manadged to fully enter a working Act 2, and even die and beat the Zone with it still being treated as Act 1. Finishing off Robotnik also activated the Act 2 title card and timer.
So get yourself into Fire Act 1.
Work your way into the spring area.
Get to the very bottom but stay visible.
Scroll to the right.
Keep going and you'll get here, Knux's Miniboss locale.
Scroll up and you'll get Sonic back.