[hurrdurr57]

I think it's mainly from the fact that they had the same problem in SD 2.0. In the interest of safety they removed so many NSFW images from the training set to the point that it could no longer generate coherent images of humans in general. This was resolved when they added NSFW pictures back into the training data for SD 2.1.

We won't really know for sure unless stability ai comments on it or releases the training data but it is the most likely explanation.

Someone will likely train this model on porn anyway in the next few months so we'll see if the finetuned models work better.