What you were expecting to happen, and what actually happened:
Cast a spell with the new troop Soul Gate. It doesn’t scale with souls, it doesn’t inflict Curse as it’s supposed to according to the description.
How often does this happen? When did it begin happening?
Always.
Steps to make it happen again
Cast a spell with Soul Gate.
Why Gate of Souls spell only uses the booster when adding armor, while it does not use the booster when draining life from enemy? Sentence describe this spell as “…drain 10 Life from them, boosted by my Souls…”