In just about every model, but let's use 538 as the canonical example, there was a reasonable chance for the Dems to lose seats but a very, very low chance they would lose control. We are looking at a two or three standard deviation variance from expectations for loss of control but losing seats was well within the range of outcomes that should have surprised no one.
I think it’s interesting too how some of the qualitative predictions (like Sabatos crystal ball) were pretty close to the outcome in the Pres race.