I don't know, but with an unclean lid, the potential is extreme. The lid accumulates water droplets that can easily drop back into the food with the bacteria included.
But we've all already been using microwaves this way for decades. If it's so harmful, then it should be easy to point to the evidense of the harm. It's not all that interesting in this case to just go by what seems obvious intuitively, because we aren't talking about a new thing and guessing if it will be a good idea or not.
