The RNG sometimes decides to randomly add sky colors or certain features, there's nothing you can do about it other than write a new age. But that by itself does not cause instability. Check that you have all the following:
- A biome distribution, exactly one symbol.
- Specific biome symbols, as specified by the distribution. (Single biome - exactly one. Checkerboard - exactly two. <Size> biomes - at least three. Natural biomes - none.)
- Exactly one of each of the following: World Generation (flat, caves, normal, skylands...), Time, Light, Weather.
- At least one of each of the following: Sky color, Fog color, Cloud color, Sunset color.
Last, some symbols cause instability every time they are in a book, no matter what. This includes Dense Ores and Accelerated, there might be more.