It's a minor glitch at this time. Give it some time and it will eventually start showing the terrain. (may have to go underground to force it to cave mode, then back out to make it realize it's supposed to be working )
I was having this same problem. Even creating new seeds wouldnt work most of the time but sometimes it would. But I can confirm the above post that just giving it time fixes it. Even if you travel a long distance it will update and show all the terrain you covered.
Try hitting random buttons, or open the full screen map and look at the button prompts. This happens to me sometimes and it's because it's set to show the underground terrain and if you toggle it, it will go back to the top view.