Newer Anthropic models have been specifically trained, presumably via reinforcement learning, to better use the edit tools baked into Claude Code, and this makes them more likely to misuse the differently-shaped custom edit tools of other coding harnesses like Pi.
Armin's theory for the regression: Anthropic has likely RL-trained recent models heavily on Claude Code's own edit tool, which inadvertently degrades their ability to correctly use differently-designed edit tools in third-party harnesses like Pi.