At the beginning i thought i lost the rewards (several Mythic cards) from Mastery Pass of the previous expansion (Final Fintasy), but then i contacted support and verified that actually i got them, even if i didnt see the showcase when i completed the levels.
Afrter i noticed (it keeps happening in the present with Edge of Ethernity) that it always happens after the animation of the Orb reward. So for some reason when i complete a level that includes an Orb and something else i see the Orb animation and nothing else. I dont see packs, i dont see cards, sleeves, etc. Sadly going into collection/decks i cant see the "new" tag too so i just dont know wich cards i received (or if i actually received them, unless i contact everytime Arena Support).
This happens in other events too, if they have orb rewards. Luckily i found a work around by going out (i.e. in the deck section) and then returning in the event i found my claimed card there. Alas I cant do the same with mastery pass

I play on Windows 10 / Steam.