What are you using for motor drivers? The usual cause of this is when the controller and the motor driver disagree on what a step signal is.
It’s possible for the step signal to be when the step pin signal goes from low to high (rising edge), or when it goes from high to low (falling edge). If the motor driver and controller don’t agree on this, you lose one step with every direction change, and scanning changes direction at the end of every line, resulting in the slanted look. It’ll be more slanted if you decrease the interval (distance between lines).