Ender 3 V3 SE - What caused this?

I've spent a whole cali rating my printer, so I've got my flow rates, z offset, print speed and nozzle temperature all sorted and the start of the print was absolutely perfect. I got about 50% through it and all 6 pieces look like this. Ender 3 V3 SE, elegoo white PLA, 0.04mm nozzle woth 0.02mm layer heights at 200C nozzle temp