It seems like the behavior between WA and MS could just be statistics saying that WA and MS always[1] vote for the opposite candidate, rather than considering a massive sudden change in the direction that one of them votes in. E.g. it's not reflecting who they vote, just who they most vehemently disagree with.
I'm not sure why that kind of interstate correlation should impact predictions?
IANAS but it feels like these correlations were added to compensate for the failure in 2016 to recognize that state A going one way implied that state B would also go that way. It "feels" like a more correct approach would be to compute some kind of error/weakness measure in a states polls by bringing in those of its geographical neighbors and incorporating the polling error of that entire block vs prior years. Or something.
The intuition I'm having difficulty conveying is that actual voting correlation is based on neighboring states only because you've got bubbles of ideology that aren't strictly cut along state lines. If strength of opinion in a bubble is going one way, then you'll see that mostly in the state at the center of the bubble, but the bubble still spreads into neighboring states, and a "stronger" bubble could push it geographically further into those neighbouring states, and/or could increase the bias in areas inside the bubble.
[1] "Always" == most recent history