What you were expecting to happen, and what actually happened: Casted Princess Elspeth on own The Banshee Queen to kill it. But instead Death Ward was activated.
How often does this happen? When did it begin happening? 20% chance when using this spell since I unlocked Astral Spire’s Death Ward node (in The Mountains)
Steps to make it happen again cast Princess Elspeth on an own troop
but you still get everything else from the spell it seems, other than the troop being sacrificed? It doesn’t appear to block the other abilities in the spell, except Elspeths summon.
But if you’re trying to sacrifice the troop to make way to summon something else (immediately in the case of Elspeth), that’s a big problem. Having a troop you don’t want, that you expected to be replaced by a new summon, can massively affect the battle.
Even if you get the other effects, you should be able to choose to avoid the effect if you want, when triggering the death yourself, if that’s part of your tactics.