It's a flat print extruded to 3mm thick.

The problem is that each layer gets infilled up to where the cut out is, then some of it has the infill started wherever it can extrude an uninterupted line. After that it fills in between wherever it left off.

It doesn't just fill in up to the cut out, do a 'Z' hop over it and start on the other side, if that helps to explain what's happening.

The layers are put down in a diagonal pattern and the next layer is put down at 90 degrees to the previous layer.