Well, using the 2215 airfoil for the whole tapered wing panel does change something but doesn't really make things better.

In both cases, 3 and 6 degrees washout, there's now a real tip stall: the elliptical wing tips stall first. The stall is not abrupt but smooth and progresses slowly - just in the wrong direction. The stall starts at the tips (aoa=12) and progresses inwards until it reaches the root (aoa=17). Duh.

I think it's simply the high taper that makes for a "bottleneck" in the midlle of the tapered wing panel. There the chord is too small while it is enough at both ends, so to speak. Seems the designer knew what he was doing.