LoR: Lux mechanic interaction

I didn't see anywhere to discuss Legends of Runeterra stuff so here we are. Sorry.

Ok so I'm playing a match and have a Lux on the field. I go to use Double Resolve on a unit, which will give my Lux her mana for her effect. My opponent counters by playing a card that kills the intended target of Double Resolve. Ok, fine, I do like counterplay. Here's my issue. I played the card, which cost 6 mana. 6 mana went away. But when the unit died, the card had no effect, and blew away like dust in the wind- AND LUX DIDN'T COUNT THAT. I don't understand.

The Lux card says "I've seen 6 mana spent." 6 mana got spent, regardless of if the card had an effect or not, so Lux should have triggered her effect.