Total anecdote, I had similar washboarding with my mk3s, I did a bed levelling mod that replaced some of the standoffs with silicone tubing spacers and that helped a lot. It was always in the same spot and it would mostly show up on prints that covered large areas and toward the edges of the bed.

I do echo others though that I do think extrusion multiplier and first layer offset do play a role with it, petg doesn’t like the same amount of squish that you can get away with using abs (not to say it doesn’t happen, just doesn’t look as rough, it doesn’t cause a failed print for me and I don’t mind a quick cleanup with a deburring tool depending what it’s for).

Does it show up if you flip the sheet just as a thought? I recall having a bit more of an issue with one side of my satin sheet for example. Last thought, could be worth giving things a long heat soak before doing meshing and homing, make sure everything is more or less stable expansion wise.