I guess this is rather category #2 than #3, but my favorite example is Advent of Code 2021, day 24: https://adventofcode.com/2021/day/24. I think many people consider this one of the hardest AOC problems ever and at the time many people didn't even code up a complete solution but solved in manually (me included). However, with Prolog it's almost trivial. So many of Prologs strengths come together here: writing parsers, writing interpreters, solving integer constraints, and in particular "reasoning backwards".
