Sounds like Jeffeb3 is right.
What are your current rapids and feedrates set at, you should have Z rapid, and Z plunge, XY rapid and feerate. If you are still using fusion360 please use estlcam for testing, I can’t diagnose the million issues that can come from fusion.
Just want to be sure this is what you have going on.
1-To start a cut have the whole machine starting at the stop blocks at the end or otherwise squared.
2-Power all three axis by manually moving them with the LCD. I usually lift it to somewhere near the cutting surface, drive my yaxis to the corner (my table axis) then drive my X axis(my rail axis) to the starting point.
3-Then make sure to drive the Y axis (my table axis) further than your whole job will be. This is important, this ensures you will not rub or bind and if you did not start square it will show here.
4- Drive it back to the start, set your z height and start your job.
You should never move it by hand, unless it is powered down and you SLOWLY drag both sides back to the starting block/table end.
I use mine (large enough for a full 4x8 sheet plus some) all the time. I slap 1/4" MDF sheets on it randomly and cut lowrider parts at full depth at about 8mm/s. For now I would not exceed that speed, more on that later. This has a lot of moving mass you have to move it slow but you can compensate by cutting deeper.
If your side plates were not perpendicular to the bed surface you would find Jeffeb3’s problem where you cut was nice on one side and stair stepped on the other along the rail axis.
The only way to get star steps along the table axis is skipped steps. From your rails dragging too much or your tension wheels pinching too hard.
Belt issues show as ovals shorter along the table axis, or bit lift marks again only on the table axis faces.