RTF is a global modification that replaces all towns in the game.

Damn, so the editor crashes the model because of too many polygons? And the same mess happens in the game? It's just not very noticeable in the picture...
P.S. It's unlikely that Polygon Cruncher will automatically create a decent retopology.

Added 1 minute ago
P.P.S. And look, in your first screenshots, even part of the throne disappeared; did you do something about it later, or not? Because now it's displayed completely.

The floor issue was my mistake; I quickly enlarged the model but didn't center it, so the floor went underground.

I used the cruncher when I was extracting the faces of the witches from Warcraft.

The larger the model, the less chance that polygons will disappear; I loaded a large model into the game.

It's not clear what exactly it depends on. For example, I worked with swords from Grim Age - there are few polygons, but triangles disappeared on the chips. What's the problem? I decided that the problem is that on the texture map, these triangles are compressed to the size of a single line.

Most often, fangs and spikes are damaged, which have many small polygons at the end; also, faces, where areas may disappear at the corners of the mouth, transitions between the eyes and eyelids, polygons in the nostrils, etc., in general, everything that is small