I've been using LR for several years but this artifact just started appearing today when I imported a batch of photos. It goes away as exposure is increased in LR. This is with a Nikon D7000. I haven't tried other cameras to see if it happens with them but suspect it's LR and not the camera as the posterization is not visible when the photo is viewed in the camera.
You've turned on the clipping warning in Develop. On your histogram there are two little triangles. Click on the left one to turn on and off the warning for underexposed areas. The right one is for overexposure clipping.
Thank you. I finally figured that out. It's not a feature that I use and
was unfamiliar with it. I appreciate you taking the time to respond.