mine got it in about 30 seconds...
Nice puzzle... I like it :)
Mate in 20
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Houdini 1.5 finds the mate in 20 in a matter of seconds.
If your engine cannot solve this, it's time to change...
It's time to change 
You're probably right.
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Stockfish 12 doesn't see a mate at all running 10 min on 10 cores....m @RHoudini
I'm correct. I even raised it to Stockfish github but the fools there started arguing it doesn't matter - then one programmer said they can't fix it.
Chess engines are not good at solving human compositions with long solutions. Puzzles made by humans have an individual logical structure which does not follow the common logics of chess games - the thing general chess engines are designed for. There is little doubt though that - with the progress of AI - they will also overtake us in that domain.
Apr 13, 2021
0
#11
Engines dont see mate vs forced mate and a puzzle must be "well composed" to be a forced mate (meaning optimal play on both sides). This is an open theory question because it is assumed that all optimal play will be a draw once chess is solved. Finding one line of mate in 20 doesnt prove its forced with all 69 trillion games each 10 ply.
This puzzle is "well composed" and there is no particular reason why an engine could not solve it as a forced mate beyond a lack of reading depth and heuristic logics. One need not solve "chess" in order to solve a puzzle.
Another weird bug on chess.com. They keep piling them on. We used to have SF15 with NNUE but now we are back in the ice age with SF11. Can no longer select any of the newer versions. I'm afraid they have gone mad again.
White to move and mate in 20.
I'll give the solution in a couple of days.