A solver fills a few rows for free, then hits a wall in the middle of the board. Here's the mechanism: a scarce piece spent in the wrong place, rows ago.
deterministic — reproduces byte-for-byte
just research-piece-theftFill the board top-left to bottom-right and every new cell already knows two of its colors: the north color from the piece above, the west color from the piece to the left. The cell needs an unused piece that can show that exact pair. Those demands are scarce, with only about three possible pieces on average, and 47 of them have just one.
So a board that looks healthy, most pieces still in the box, can already be doomed. Somewhere back up the board, the single piece that could ever serve an upcoming cell was used for something else. When the solver finally reaches that cell, there's nothing to place.
The engine fills a board to a cell with a single legal supplier; steal that piece and watch the cell die with the box still full.
A tempting fix is a global check: do the remaining pieces still cover the remaining cells? It doesn't help. Globally the supply is fine; the failure is one scarce piece misallocated, not a shortage. So a global lookahead sees nothing wrong right up until the cell turns out to have no server, which is why this wall resisted so many attempts to prune it early.
Set this beside no forced moves and the trap is complete. Every piece has dozens of places it could go, so the solver is never told where a scarce piece must be saved, yet each scarce piece has exactly one demand it must be saved for. Freedom to place, no guidance on where to save.