same, it says they unlock as you get through the levels BUT some of them don't show up in your deck builder after. I just "unlocked" the thanos cards yet loging in days later they're still not there.

Came here to report the same issue.

I also haven't received my cards. I was playing on iPad, which is unusual for me. Evidence: I have 4x collected on two non-common cards from Spiderman and Avengers combined. One achievement should have given me multiple playsets.

Edit: Apparently the cards earned in the rewards are Brawl-only, but listed as "4x" in the achievement rewards suggesting you'd be earning a whole playset as if that matters for brawl. Or I guess alchemy? Anyway, I still feel ripped off but I also admit there's no bug.