No, that the curve starts down only indicates that the bullet is creating volume faster than this volume can be filled by the burning of the remaining powder. It does not prove gas production has ended.
Now, if there is no more gas (and energy) being released, then the pressure vs. volume curve will closely follow a shape of the form P*(V^k)=c. Plotted against volume, this curve is strongly concave upwards. We're looking at P plotted against time instead. Here, the bullet's acceleration will stretch out the curve, but I'm confident (which doesn't mean I'm correct!) it cannot invert the shape of the curve.
The RSI data shows a convex curve after the pressure peak.
