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

No, of course not: a bug is the disappearance of an upgraded stack when a hero flees.

Why do you think this is a bug?
Only something that makes it impossible to continue the game normally can be confidently called a bug. Or something that seems incredibly illogical (like, for example, increased damage from towers when the "Armor" skill is present).