The word “raw” may not be accurate here. Depending on the composition of the filling, it could simply be that the bottom crust is “poaching” in the moisture of the filling instead of baking in the dry environment that allows browning and flaking. If the bottom is soaking up juices from the filling, leaving it in longer won’t do a lot of good.
Most pie fillings have some way of binding the water so that it isn’t free to soak the crust, like cornstarch for fruit fillings, or eggs for custard fillings. This is also why it’s better to cook fruit (typically with the starch) before putting it in the shell, so that the juices are rendered and thickened away from the crust.
BTW, baking closer to the bottom of the oven will concentrate more eat on the bottom and less on the top.