This is the expected interaction between Vraska, Betrayal's Sting and Innkeeper's Talent L3, which your opponent has in play. MTGA has handled this correctly.

At level 3, Inkeeper's Talent says: "If you would put one or more counters on a permanent or player, put twice that many of each of those kinds of counters on that permanent or player instead."

Vraska, Betrayal's Sting's ultimate, costing 9 loyalty, is "If target player has fewer than nine poison counters, they get a number of poison counters equal to the difference."

When your opponent played Vraska, they started her with 12 loyalty because, when she "enters with" 6 loyalty, that means your opponent is putting 6 loyalty counters on her, which Innkeeper's Talent doubles. Then they spend 9 of that loyalty to use her -9 ability, targeting you. The effect calculates that your opponent should put 9 poison counters on you, which Innkeeper's Talent doubles, so your opponent puts 18 counters on you. Then you lose instantly.

Two-card instant-win combos are just one of those things that can happen at six mana. Either win before your opponent gets there, or find ways to remove the enchantment, or use counterspells to keep Vraska from entering, or just lose. It's, uh, not the most absurd combo MTG has ever had but it still doesn't feel great to play against.