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Urgent fixes are needed in the beta regarding heroes. In addition to the fact that heroes from campaigns and Mullich appear, today, on day 131, a hero with 1 nymph appeared in the tavern in the right slot (similar to the War of the Throne mechanic).
However, the disappearance of an army on a hero (presumably, it doesn't matter whether it's in the tavern or with the player) with mirroring is unlikely to be related. The nature of this bug is not yet clear to me; if it suddenly repeats during a replay, such a save would be very helpful.