Unfortunately, ACES 1.x is not fully invertible. Alex, Scott and others can give you the technical reasons why, but there were some modifiers on 1.x that wouldn’t let you invert what you sent through it. One of the examples that always got us at Disney was using images that were sRGB (like title graphics). You couldn’t invert it through the inverse output transforms in ACES 1.x, overlay it on some ACES EXRs and then get the same colors back when you put the output transforms back on.
But the number one item that we collected during our “listening tour” was that the new ACES 2.0 had to be invertible. And thus, we finally have it! I hope that makes sense.