It's as intended.
Since you've killed Skadi++ and her final form, now you'll be eligible to fight against Noa which is a random encounter.
It works like that :
Encounter Skadi, kill her, then you can encounter Boreas (which is a bonus boss that you oneshot).
When you killed Boreas, you can trigger the Icicle boss and kill her will earn you nice rewards (HR cards)
To fight the hardest one, Noa, you need to kill the Final Form of Skadi (which you did) and then you have the chance to trigger her fight ! But as Boreas and Icicle, it's random... So keep questing and you'll get her ! (Rewards are even greater, but she's far stronger than final Skadi form !)