I've run the third encounter of Spire twice, once on hunter and then once on warlock, and both times as a duo for the all solar triumph. The first time my duo got the triumph completed and I didn't, and the second time around it didn't work again. Initially we ran prismatic and swapped to solar before the encounter finished, and the second time we ran solar all the way through. We were also able to complete the first encounter of ghost of the deep on all void and the first step of that triumph completed, so I'm not sure what's up.