Realm Ore has only been sold for Proofs of Torment recently, at least 5 times already (incl. Jul 25-27 and July 16th).
However, I've seen Silk being sold for Torment on 27/07 and for Melancholy on 28/07, so it's not like materials are strictly "tied" to a certain proof type.
It seems that the issue lies in the materials lists for each day. The lists are very similar to those from Anguish 1.0, but tweaked a bit to include exactly 8 materials per day. The thing is, the order didn't matter then, but it does now. The same position means the same Proof type cost, since we have an order in a guild — first mats are sold for Agony, last — for Torment. And that's exactly what's happening with Realm Ore — it's quite often in the end of the list. It'll probably be sold for Proofs of Torment again on Aug 7th, Aug 28th, and so on (based on Anguish 1.0 lists from Orna Guild Forecast).
So my suggestion is to randomize/balance the order of materials in the lists, so we can actually buy resources with different tokens.