When you fly quickly, as you can in Spectator mode, the unloaded areas appear to have an Overworld atmosphere. Namely, the red (or new biome-specific) hue is gone and it looks like the normal white one might see in the Overworld. As the chunks have not been loaded, obviously the biome has not been decided. So, a filler or default color (say, red) should be added. 

Here's a video showing the issue: https://youtu.be/Bu1YlLdeuYw

Thanks for Jetstreamjoejoe for giving me permission to share this bug he found.