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This turned out to be surprisingly easy to fix. The fix will be included with LV 2018 SP1. The problem is that there are a very few nodes in LV that reach downstream to get their types when they think it is safe to do so as a way of giving better default types for terminals. But when the downstream terminal is something polymorphic, that has to be limited or disallowed. I never knew about that feature of LV (though I've unconsciously taken advantage of it many times, it turns out). I just needed to add a downstream malleable VI to the disallowed list so that the malleable is always driven by upstream instead of driving upstream.