Oh yeah, I'd noticed that there was an issue with this when I was inputting all my travels over the years into my list file a few months ago, but I'd since forgotten about it. Thanks for reminding me! There's also another place where this is an issue: the WA 20/ WA 9 concurrency in Sedro-Wooley. So I'm going to have to fix these and perhaps check through all of WA's multiplexes and make sure there aren't any other problems.
EDIT: After looking on the highway browser some more, I've also noticed some other issues. Will post in another thread.