Paladin Oathkeeper Vistani Set Bonus

In an effort to add to the debuff pool for ToMM runs, I've set my OP heals up with the Vistani and Apocalypse sets. Apocs set works fine, can generally maintain 5 stacks at all times outside of spreading mechanics in P3. But I can't seem to find a power to proc the Vistani set. The set bonus reads:

"When you cast a damaging AoE but it only hits 1 target, the target will take 5% more damage for 5 seconds."

Set myself up at the solo test dummy in Caer Konig and I've tested the at-will Radiant Slam along with the encounters Burning Light, Bane and Banishment with none of those powers applying the debuff on the dummy's bar. As the tool-tip reads, all 4 powers "should" proc the set bonus. Any ideas or is this something that should get submitted to bug reports?

@nitocris83