Documenting disputed borders or borders in war zones seems tricky in general. Not only can they be constantly shifting, but even at a fixed point in time, the boundary is probably wildly different depending on which of the belligerent parties you may ask - or in extreme cases, a party may deny the demarcation exists at all.
Disputes are not such a big problem, since you can have multiple independent versions of the boundary with claimed_by= and disputed_by= tags differentiating them. But you do need to find out somehow where the claimed boundary even is. Like, does the Wa State have an Office of Cartography where you can ask for the latest official maps? I haven't looked super hard, but I doubt it exists.