Your profile says you have a 100w laser, which means it isn’t a diode laser. A CO2 laser should not be generating a rectangular beam. You have a problem with the output mirror of the laser tube or have debris or other obstructions along the beam path. If you fire a pulse at the first mirror with a target paper in place, do you get a circular burn spot?
I’ve seen images someone took of a back-flashed mirror on the laser output. Somehow, the laser reflected into the tube and burned a portion of the laser tube. I can’t find it now, of course, the brain cells resist time travel of this nature.
You could also have a bad alignment from mirror 3 to the bed, causing the laser to bounce off the inside of the nozzle tube.