Sharp corners gets randomly rounded

I have built a hotwire styrofoam cutter from two eleksmaker.

I cut rectangular shapes from g-code
Some corners get rounded and I cannot understand why.
The g-code runs only one axis at the time and to get rounded corners both axes need to move, right?
Anyone here how have experienced the same thing?

Do you think it could be due to friction?
Not enough current for all the motors? I run two motors on each stepper driver…

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