Monsoons provide vital rainfall for billions but are hard to forecast. Using a next-generation climate model, we simulated monsoons at different grid spacings. The model captures key seasonal patterns, but finer grids do not always improve accuracy. They can worsen predictions by overproducing intense rain, as they artificially strengthen weather systems like monsoon lows and waves. Our work shows that smarter model physics is needed for reliable future forecasts and climate projections.