Flipping one face texture on a cube...

So, I’m using the basic cube to create a tilemap. What I found is that assigning the same texture to every tile is seamless, but looks like garbage at any distance and obviously tiled. So I created a really big texture and I have the tiles take a different part of it based on their grid position. Basically one tiled image is actually built from a 5 by 5 group of tiles. Works great… except in one direction. I have one face that is horizontally flipped so the texture images don’t seamlessly match.

So, I pulled the UV’s for the side, flipped the x coordinate, verified that they did, indeed, get changed by sending the coordinate through a debug.log before and after… run the program aaaaaand… no change. Still flipped. Used a blank tile texture with “TOP” printed at the “top” to get my bearings and it doesn’t move. I can change the UV coordinates to anything I want and it remains completely unchanged. I tried random floats just to see if it was working at all. You know… get chaotically randomized pieces of texture just to obviously show that something was getting through. Nope. Nothing.
Its hard to see, but the wall on the left is what I’m trying to change. I included the debug output to show that, yes, the new vectors are being applied to the UV.

Where is this going wrong?

void Start()
        var uvs = GetComponent<MeshFilter>().mesh.uv;
        Debug.Log("Old UV 20 coordinate: " + uvs[20].x + ", " + uvs[20].y);
        uvs[20] = new Vector2(1, 0);
        uvs[21] = new Vector2(0, 1);
        uvs[22] = new Vector2(0, 0);
        uvs[23] = new Vector2(1, 1);    
        Debug.Log("New UV 20 coordinate: " + uvs[20].x + ", " + uvs[20].y);

You need to assign the uv array back to mesh.

GetComponent<MeshFilter>().mesh.uv = uvs;