I was suspecting this myself, but now I make sure the sheet surface is at the same distance from the head at every corner before cutting, but I still get the burning in some areas.

So far I've mostly cut acrylic.

And yes, the melting seems to happen in the same XY place, although when I tried to cut a small piece as a trial in that place, that turned out perfect. I noticed that there is a bed rail running across, below the honeycomb bed so I was wondering if that could be restricting air flow so the vapours can't escape from underneath the sheet? Could this affect the finish? But then, these rails run across the middle and the bottom of the bed too and that doesn't seem to affect the finish.