I have saw into the RifleReloadProneM4SSAS class which correspond to the Prone reload action but unlike the other, it leads to the same route as stand position.
In "\A3\anims_f\Data\Anim\Sdr\gst\", there are a bunch of rtm, but for example, there are a GestureReloadM200.rtm (stand animation reload action) and GestureReloadM200Prone.rtm (Prone animation reload action).. But there is no a GestureReloadM4SSASProne.rtm animation!
So I think it would can fixed easely by creating other animation called GestureReloadM4SSASProne and edit the config properly by editing the previus route to:
class RifleReloadProneM4SSAS : RifleReloadProneBase { //["Default"]
file = "\A3\anims_f\Data\Anim\Sdr\gst\GestureReloadM4SSASProne";
leftHandIKCurve[] = {0,1,0.01754,0,0.97453,0,1,1};
speed = 0.105079;
};