HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Is there any point in arguing about this? Or are you just bored? Well, okay: to me and many other players, the disappearance of the enhanced stack when the hero leaves seems incredibly illogical. It's like if, when I get off a bus, a few more passengers suddenly disappeared from it. :) Is that clear?

Well, when someone disappears in real life, it's illogical, but in a game, it can happen any number of times.
Why I think it makes sense: the appearance of an enhanced stack is a strengthening of the neutral creatures, and the disappearance is a weakening. So, you could say it's a balance.

Added 20 minutes later

Is there any point in arguing about this? Or are you just bored?

You expressed your opinion about this here - and I'm not blaming you for doing it out of boredom. As you yourself said - "for the sake of fairness."

Added 46 minutes later

Then the only option is to add an extra option in the built-in XB rules in HD to enable the bug...

Why not make all HD+ features optional - after all, there are probably different opinions about most of them?

And I don't think it's a big problem for players to agree on this. After all, in the case of tournaments, the organizer simply sets it rigidly...