that is because you currently are using Ngons, N being more than 4 corners.
You need to add more lines to your mesh.
Select two vertices (vertex) and press the “connect” button in editable poly or editable mesh mode.
Try to make sure that you dont have polygons with more than 4 corners.
also, you might want to create a UV map for the materials to apply properly.
You need to model without convex N-gons. At the moment when 3DSMax triangulates the mesh, it will do so in a way that creates trianges across the areas you’ve cut out because it doesn’t know any better.
And it would be even better if there was another vertical line in the middle, because having all those corners cramped towards one vertex is actually less optimal for dynamic light.
Most of the time it stays rather identical but occasionally it does happen that you need to unwrap it again. it shouldnt be that hard or timeconsuming though