Thick layer in the middle of print

As one of my first prints on my P1S with AMS, I printed the Bambu P1S Poop Chute remixed by Broman with black PLA filament. The print came out fine but with one thick layer all the way around in the middle of the print (see pictures). As a newby to 3D printing I have no idea how this can be possible. All the rest of the print looks fine.