What allows this? I thought planeswalkers return with printed loyalty?
Your thinking is correct, when Teferi, Hero of Dominaria returns from exile as a result of Banishing Light being destroyed, it is a brand new object that should be exactly as if it was being played for the first time.
400.7. An object that moves from one zone to another becomes a new object with no memory of, or relation to, its previous existence.
There are several possible effects that could cause Teferi to have entered with 8 loyalty counters, but no way to know for sure what happened in your particular game unless you know the entire board state.
One simple possibility is that your opponent could have had Doubling Season or Vorinclex, Monstrous Raider or a similar card in play, which would cause their planeswalkers to enter with twice the normal amount of loyalty counters.
They also could have activated an ability such as Gilder Bairn as soon as Teferi entered play.
This has happened to me on many occasions, and although people have come up with perfectly legal ways for this to occur, it is actually a bug with planeswalkers exiled with prison realm/banishing light. Please see this link (and countless others) where it is confirmed by WOTC that this is a bug on arena which should be fixed with the Strixhaven update.