I've been reading through this thread, thinking about a little tidbit that could have some effect, and I was happy to see it fit with the WoB.
We know that "static" Investiture resists interference by "foreign" Investiture (for example, Wax being able to tell Spikes and Metalminds by the difficulty to push them). Could this not be the reason that the spike interferes with Harmony's searching? I mean, we obviously don't know the details of this, but it seems fairly intuitive to me that this would be the case. To be totally clear, I'm suggesting that the unverified Shard's God Metal would contain Investiture that would make it difficult for Harmony to find. Then again, do we see this with Preservation and Ruin? Is Vin able to see the Atium when she ascends, or are they already burning it? Thoughts?