As the wire melts through foam it meets resistance and bows backward in the middle, lagging behind the carriages — like a bowstring. The faster you feed or the cooler the wire, the more it lags, which rounds off corners and makes the centre of a cut trail the ends.
Reduce it by (1) raising wire tension so it resists bowing, (2) balancing heat vs feedrate so the wire melts foam just ahead of itself rather than being dragged, and (3) slowing down on tight detail. Some software adds lead-in/out and corner dwell to let the wire catch up. On long spans, higher tension and a touch more heat at slower feed is the cure.
Reduce it by (1) raising wire tension so it resists bowing, (2) balancing heat vs feedrate so the wire melts foam just ahead of itself rather than being dragged, and (3) slowing down on tight detail. Some software adds lead-in/out and corner dwell to let the wire catch up. On long spans, higher tension and a touch more heat at slower feed is the cure.