The usual way to crack a logic puzzle is to find a spot where only one piece fits, place it, and repeat. That lever doesn't exist here: every interior piece has between 73 and 137 possible neighbours, and not one is ever pinned to a single option.
deterministic — reproduces byte-for-byte
just research-no-forced-movesEvery one of the 196 interior pieces has between 73 and 137 other pieces that can legally sit to its right (counting right-hand partners, as the chart above does). The typical piece has over a hundred options. Not a single piece is ever pinned to one choice.
This is the flip side of forbidden patterns. There, almost every combination of pieces is impossible. You'd think all those rules would corner pieces into place. They don't: the constraints rule out combinations without ever cornering an individual piece, so a solver never gets a free, forced move to build on.
The 196 interior pieces, bucketed by how many right-hand neighbours each one accepts. The whole distribution sits far from one.
The engine fills a few cells; then we count, live, how many pieces legally fit the next one. It almost never drops to one.
Put this beside forbidden patterns and the real shape of the difficulty appears. Locally the puzzle looks loose: any piece fits next to plenty of others, so there's nothing to propagate and no chain of forced moves to ride. Globally almost every combination is illegal. The hardness lives in that gap: lots of local freedom, almost no global consistency. A solver has to make a long run of free-looking choices that only turn out wrong much later.