It doesn't seem to have anything to do with image size or any other external assets. Turning off georeferencing for the terrain or disabling the water shader in the ocean planet object both seem to resolve it, as does changing the ocean planet radius to where the water is not visible. So that suggests it's something to do with the water rendering, or interaction between the water shader on one planet and the terrain on the other. I'll send it on to dev and see if we can figure out what's causing it, but it's definitely not normal. In the meantime if you don't need the transparency effect of the Water Shader, you could just use a Reflective Shader which does not seem to cause the issue.
- Oshyan