I guess this may indeed be a z issue, those diagonal lines could be when the nozzle moved from corner to corner without laying material down.

I don’t think this has anything to do with the material or temperature or anything like that.

I’m not overly familiar with the Ender, but maybe run a bed levelling protocol or other calibration option? The plate is probably toast unfortunately.

Edit: did you switch plates between the PLA and petg? They might be different thicknesses if so.

What printer? Is this a residue that is raised, or a depression on the plate, or just a surface finish reflectivity change?