Jagged lines are a notorious issue on ipads. My understanding is that if you draw diagonally and slowly, you are far more likely to get jagged edges than if you draw the same diagonal and more quickly. Also, I have seen jagged edges on certain apps on certain ipads (including but not only Procreate). At one point this issue was so bad that you could not draw circles.. they came out like octagons. Apple, of course, never admitted to the issue, but since so many of us have seen it across so many apps, it's definite that they are at least part of the problem. IPad Air 2 is one of the worst iPads for artists because of this problem. But it is not alone.
Putting the dots together, if Astropad is slowing down a brushstroke that is diagonal, I could easily see how that would result in jagged edges.