Hey look! The search feature actually produced the exact topic I was looking for!
I was playing through this game again recently, and upon getting 30 stars, got the normal message about Bowser's door being accessible now..
"They say that Bowser has sneaked out of the sea and into the underground. have you finally cornered him?"
The first Bowser level was in the underground, and Bowser level 2 is in the Fire Sea. Could it be possible that the first two Bowser levels were supposed to be switched?