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

baratorch, take a look at the save file. It seems there's still a bug in Quick Battles.

We were playing for fun in an alliance when Sir Mullich's computer attacked and lost to a human hero (Galtran) in a Quick Battle. The human decided to replay, and this time his computer lost!
In a regular auto-battle, the computer couldn't repeat that result (win). I don't understand how it managed to fight so well in the Quick Battle?

So, let's load a hotseat game with Quick Battles enabled and pass the turn. We need Sir Mullich to attack (we'll wait a week for Titan). Let's keep reloading until he attacks.
Then, next time, we'll disable Quick Battles and just have a normal auto-battle.

It took me three tries manually to win :D

mysterious_quick.zip

I looked into it. First, I made the Quick Battle replays possible and added quick completion by pressing Q during another player's turn in hotseat mode.
Second, I conducted research and discovered the following:
In Quick Battles, there are no obstacles on the battlefield! Therefore, after tactical positioning and an angel attacking, the defending angel moves to hex N21 on its first turn – this is how the battle starts in Quick Battles.
However, in the same battle in visible mode, there are obstacles on cells N20 and N21 that protect the archers from being attacked by angels.
Therefore, in Quick Battles, the archers are blocked from the very beginning of the battle and suffer heavy losses, while in a regular auto-battle, the skeleton stacks receive significant damage from ranged attacks until the end, which ensures the defeat of the defender.
That's what happened. We will think about it and fix it.
And yes, the battle can be won manually despite the obstacle.
Thank you for helping identify the bug.