One way to check is to turn on the Projected Volatility. We can pretty much eye ball the shape of the projected IV curve. From the look of it, the project IV is pretty much linear. The curving up part above the market has been taken out. I think why the new model is so inaccurate is because the it assumes the curve-up part will stay static. In reality, when market turns-around and start grinding up, the curve-up part will flatten and crash much faster than IVs below the market (the linear part). Therefore, options above the market will be over-priced by the new model. Especially for butterflies, if the right leg steps onto the curving-up range, for sure it will crush far more than the other legs. As a result, if the model assumes a static skew, it'll be highly inaccurate. Not 100% sure though. Just my educated guess from the limited explanation of the new model in the release notes.