Seeing no one still didn't get the answer... I'll explain why it isn't what you guys thought:
- ❌ Hikaru Nakamura: Reached World No. 2 and never became World Champion, but he has never lost a World Championship match after leading by two points.
- ❌ Fabiano Caruana: Reached World No. 2 and played a World Championship match, but the score was tied throughout the classical games.
- ❌ Akiba Rubinstein: One of the strongest players never to become World Champion, but he never lost a World Championship match after leading by two points.
- ❌ Ian Nepomniachtchi: Lost two World Championship matches, but not after leading by two points.
Bonus Hint
This grandmaster is from Hungary, and the World Championship match in question was against Veselin Topalov.
That should narrow it down considerably.