When night comes the sun disappears like it's supposed to, but if you look in the direction the sun is located, it makes the sky at the bottom and clouds disappear, and only the stars are seen. If you look away from the location of the sun, however, the sky looks normal.
When the time is at 12000 ticks, the sunset looks normal, but at around 13000 ticks, the bugs starts to become noticeable. This is also the time where the sun appears larger in the sky than usual. At 14000 ticks the bug is very noticable, and can be seen al throughout the night, as long as you look in the direction the sun is at.
This bug also appears in the Nether. Because there is no sun in the Nether, the bug always appears when you look straight down.
How to replicate
In the Overworld, simply turn the time to night, and look in the direction the sun would be located. In the Nether, simply look straight down. This bug doesn't appear in the End.