https://ibb.co/yqsrqh8
There is a button on your View ribbon bar/menu under the name settings So to toggle it back off its View > Settings > Wireframe Rendering.
Watch this video, it's a new(ish) feature of the studio. https://www.youtube.com/watch?v=ge9YeBKQpoA
Hope this helps!
Try maybe fixing some properties of Terrain. I think it'll fix it.
Edit: If it doesn't fix it, then you may have to re-install Roblox Studio.