It's not a bug; it's a feature!
Seriously, four minutes of dialog were recorded for Manny and Domino in year three but for whatever reason it's not accessible in the game. I don't believe it's a bug. Instead, I think it's possible they decided that the dialog spelled things out too clearly too soon. If the decision was made at a late stage, that would explain why the whole four minutes were cut instead of just the spoiling parts (a similarly spoiling exchange between Manny and Sal was cut from year one).
It's still possible to hear the dialog. You could get Scumm Revisisted and find it for yourself, or you could give me an e-mail address and I can send you the mp3 I made of it years ago.