user33246: Chess.com - Praggnanandhaa wins 2022 Reykjavik Open:

he missed a unique underpromotion to a bishop as the only move to win the position

Why does a Queen promotion not win him the game?

user4418: 
Why does a Queen promotion not win him the game?

Because black can play the kamikaze Qg2+ forcing stalemate after Kxg2.
Here is the critical position in the game where Hjartarson could have forced through the pawn to promote via the queen sacrifice Qf7+ but needs to promote to a bishop. Hjartarson instead played e7 leading to a draw. Presumably he calculated the queen sacrifice and saw that promoting to a queen led to stalemate but didn't calculate the underpromotion.
[title "Hjartarson vs Andersen Reykjavik Open 2022"]
[fen "4r1k1/3R2p1/2q1P2p/5QnP/5N2/6P1/5P2/6K1 w - - 0 1"]

1. e7 (1. Qf7+ Nxf7 2. exf7+ Kh7 3. fxe8=B! (3. fxe8=Q? Qg2+ 4. Kxg2=))


33246: <p><a href="https://www.chess.com/news/view/praggnanandhaa-wins-2022-reykjavik-open" rel="noreferrer">Chess.com - Praggnanandhaa wins 2022 Reykjavik Open</a>:</p>
<blockquote>
<p>he missed a unique underpromotion to a bishop as the only move to win the position</p>
</blockquote>
<p>Why does a Queen promotion not win him the game?</p>
 4418: <blockquote>
<p>Why does a Queen promotion not win him the game?</p>
</blockquote>
<p>Because black can play the kamikaze Qg2+ forcing stalemate after Kxg2.</p>
<p>Here is the critical position in the game where Hjartarson could have forced through the pawn to promote via the queen sacrifice Qf7+ but needs to promote to a bishop. Hjartarson instead played e7 leading to a draw. Presumably he calculated the queen sacrifice and saw that promoting to a queen led to stalemate but didn't calculate the underpromotion.</p>
<pre><code>[title &quot;Hjartarson vs Andersen Reykjavik Open 2022&quot;]
[fen &quot;4r1k1/3R2p1/2q1P2p/5QnP/5N2/6P1/5P2/6K1 w - - 0 1&quot;]

1. e7 (1. Qf7+ Nxf7 2. exf7+ Kh7 3. fxe8=B! (3. fxe8=Q? Qg2+ 4. Kxg2=))
</code></pre>
