A hotfix for this was just announced on Twitter, just tried again now and I could claim the pack with no issues. The event items are back on my game too now, the gear will have to be upgraded again but that's no problem.

I've had this happen too on PC, I tried to claim the pack but received none of the contents. Instead the existing Yule festival event content including the the Mōrganiht Ceremonial Set I had played through the event to get is now gone across all my saves. I don't think I had any runes slotted on my set but I noticed the game give me a few Ingots, Titanium, a lot of leather and Iron suddenly so it seems the materials I used to upgrade them came back to me.