Some hotfixes have just been pushed to live servers which prevents the Diablo Event charms from working in Raids and Dungeons.
Diablo Event Charm Hotfixes
Large Charm of Intelligence – No longer functions in Rated PvP, Raids or Mythic+
Large Charm of Strength – No longer functions in Rated PvP, Raids or Mythic
Large Charm of Dexterity – No longer functions in Rated PvP, Raids or Mythic
Serpent’s Grand Charm – No longer functions in Rated PvP, Raids or Mythic
Small Charm of Life
Mana nerfed to 2,400 (was 12,000)
No longer functions in Rated PvP, Raids or Mythic
Small Charm of Inertia – No longer functions in Rated PvP, Raids or Mythic
Stalwart’s Grand Charm – No longer functions in Rated PvP, Raids or Mythic