That’s caused by the Bing Maps tiles (in this case) using the Web Mercator projection, which stops at ~85 degrees latitude. When a terrain tile is at least partially covered by a texture, that texture will get stretched out to the edges. But if a terrain tile doesn’t have any imagery tiles at all that overlap it, you’ll get that white, untextured look.
We have an issue covering this, but it hasn’t been a high priority so far:
