Ar plate is known for it's irregular hardness due to it being a recycled plate. I think what you are running into is fluctuating material properties across the plate. We cut it with a HPR260 fairly regularly and it's never and issue but we're probably running a bit more heat than your setup.
If you want consistant plate, try Hardox and see if it's better. Ar400 or Hardox 400 have the same spec's but Hardox is supposed to be a more consistant plate.. usually more expensive too...
No association w/ Hardox or their suppliers other than i've used it and like it..