Print fine of front, but weird on back

Hi everyone.

I recently upgraded from a cr-10 to CR-10 V2. Did a couple of small test prints.

They worked fine. But now I've printed a larger object (a head model). The side facing the front of the printer was nice and smooth, but the back has issues. Right down the middle there's a weird seam, and everything on one side is good, but on the other side very poor.

Never seen this before, so I was wondering if anyone could help identify the issue.