From prestige 2 through to prestige 6, I have seen strange cards (from higher unlocked tiers) appear when using "summon" cards (Caprice, Thieve's Instinct, etc.) I was happy to have some of these cards, but as they had not been unlocked, I am guessing that this is a bug. I think that I have unlocked 6/7 or 7/8 tiers, but when I only had the first tier unlocked, I had some OP cards appearing. This never happened to me in the main branch.
I don't actually know the design intent, so just alerting you to the fact.
Use "summon" cards during a playthrough and see what cards appear