Generally speaking warping in a PCB occurs because of a mismatch between the amount of metalisation (copper in this case) on each of the layers. So for a simple two layer board, if your bottom layer has more copper than the top layer, the expansion of the bottom will be more pronounced relative to the top layer, creating a imbalance and consequently a degree of warping. Now having said that, the amount of imbalance, the thickness, type of material etc all play a role here, so the amount of warping and twisting may vary etc. It becomes even worse with increased area of a PCB such as with a panel.
The best and simplest permanent solution is to reduce or eliminate this imbalance of copper metalisation so that expansion during heating is equal on all layers of the PCB, in all directions. You can achieve this by adding copper pours that are part of the ground planes, or just simple copper pours left floating, that just serve the purpose of equalizing the metalisation level on the respective layers where it's lacking.
Your question assumes we know what type of PNP machine you use, how your boards are supported, and many other variables we cannot even imagine as you have not supplied any photos of your setup. In the end it may not even work for you and will lead to a possible worse outcome, than just fixing the source of the problem.