Anyone else notice this? If I look to the left or right I cannot see the textures disappear but if I look straight I can see it out of the corner of my eye. Very annoying 
Draw distance doesn't affect it. I've made a bug report with video:
https://forums.frontier.co.uk/showthread.php/295008-Planet-visibly-draws-in-periphery
I see the terrain pop-in too, most often when approaching planets. Orbital glide is the worst offender. I haven't found a setting yet (terrain detail, terrain work etc) that stops it.
I don't think its terrain pop-in exactly. I notice this too, and only on the edges of my peripheral vision. It's like at a certain degree of head turn away from the planet there's suddenly a bunch of black boxes, but when you look back everything looks fine.