Given that the private key range is known for each puzzle, I am not sure I understand the advantage that someone will have once the public key is known.
For Pollard's Kangaroo, you need to know the public key that you're trying to match.
OP was theorising that once they publish their transaction, someone could use Pollard's Kangaroo to trivially solve the private key in a matter of minutes and then publish their own transaction stealing their prize.
I'm not overly familiar with the performance of this particular algorithm or the available scripts for it... but if the actual winner just disables RBF and sends with a "decent" fee, the odds of their prize being "stolen" would be pretty minimal, I would think.
that one Tesla V100 can check 715 M keys per second by using bitcrack. Assuming you can get google to rent you 176k V100's