Can I refight Ring Fit Adventure's final boss?

It seems you cannot, according to dafipp on Reddit:

I wasn't sure so I just went back and checked...No, the original Dragaux fights do not open back up after completing the first run thru of the game.

There is however an option to

start a New Game+ or New Game++, and you will encounter him there again

It seems that that the answer from Mathias is no longer correct. You definitely can refight the final boss again (I've tested it myself).

Since it seems that not everybody can I have the following 3 hypotheses.

• You need to complete all stages for each of the 23 worlds.
• You need to complete all stages and secondary missions for each of the 23 worlds.
• You need to get the most recent update.

I remember that when I finish the game the first time I saw the exclamation flag on the final boss when I got back. That leads me to think that the first hypothesis is the correct one.