Maybe someone can offer some advice.

I tried to mess with the path to the hero model... the result was predictable.

Haha, well, that's understandable; the game doesn't know what you want it to do then. I didn't break it; I just pointed to a different path for a different hero, and the effect is the same – invisibility. Although, I need to add one nuance. The hero isn't visible on the map, but if you go into the hero's stats, look at his army or artifacts, he shows up.

Added 6 minutes later

I was able to achieve this bug! To do this, I took a working mod, found the model of the red hero, and deleted it – voila, the hero disappeared! At the same time, only the red hero disappeared; the other players' heroes remained.

I achieved this bug before when I was setting up the hero's attack in battle; I indicated the wrong symbol, and the hero became completely invisible on the map.

But here, it's a little different. It doesn't matter which hero it will be in the end; if I remove this one from the game, the same thing will appear on another one. If I remove that one too, it will move to the third one, and so on.