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It can be saved in 9 out of 10 cases; if you stand on the teleport on the first day and don't move further, that terrible hero won't jump out of the portal. On the second day, take the town and move behind the teleport, end your turn. After that, the hero arrives, takes the town, and is killed by the event. On the third day, you take the town and preserve the ability to run around with your main heroes for more than 7 days without a town, plus an overpowered hero with an army is waiting for its time.
One question: why? According to the logic and plot, the author clearly did not intend for this to happen. Using this opportunity allows the player to gain 2 huge stacks in their army after exploring the portal, which they shouldn't have, and makes the rest of the map much easier. Essentially, it's the same as playing with cheats, and then the question is: why not just play with cheats? It will make the game even easier.
And by the way, Season isn't to blame here; this kind of behavior from the AI actually occurs because of HD+, without it, the AI always attacks, even if the hero is standing on the portal. From this battle on the Aeris map, we can conclude that quick battles are conducted differently in HD and HD+:
https://www.youtube.com/watch?v=DC32SHQrBvA (watch from 9:00)
Most likely, not only are the battles conducted differently, but the AI also evaluates its strength differently before them, which is why it doesn't attack with HD+ enabled.