Hello,
3ds max changes polygon ids when I assign a material to the selected polygons. For example if I have 2 ids and assign identical material to both ids, my ids become 1. Or for example if I have 4 ids and assign to id 2 a texture that is the same texture as the one on id 3, then id 2 becomes id 3. It is very disturbing. Material ids are hardly made asset defining the geometry, so why max considers them as a subjet to change at any spot?

I think their priority is keeping the consistency of material ID with material slot (in the material editor), such that if you change a polygon's material ID the polygon's material will instantly change as well.

If you want to keep the "same" material in two different IDs, go to the material editor and drag that material to an empty slot. Every slot holds a different material value, even if you dragged one there - they will be different materials even though they have the exact same look, but now each one will have an exclusive ID. 

Because these IDs point to sub-materials of a Multi\Sub-Object Material, so 3DS Max automatically detects that you're trying to assign the same material to different polygons and changes the polygon's ID to match the sub-object element.

After you apply more than one material to different polygons of the same mesh, the main mesh's material becomes a hidden Multi\Sub-Object material. You can edit this hidden material by selecting an unused slot in the material editor, pushing the eye-dropper icon (named "Pick Material from Object"), to the left of the material's name, and picking your mesh in the scene. This slot will then represent this mesh's Multi\Sub-Object material, and each of those ID's you see in the list will correspond to polygon's material IDs.

I agree it is a bit unintuitive the way it works, but since there's no easy way out of this I suggest you clone materials (drag a slot to another slot and rename) and find other ways to do what you want.

I have solved it by just drag-droping the texture file straight from the directory onto selected faces. So I can view and edit uv mapping on a given texture, luckily this does not redefine my mat ids.