It looks like it actually is emissive, as I set the TempMapMaxTemp to 0 and the red disappeared:
However, I don't understand why. It is way too cold. Also, the atmosphere appears to be partially pink, while it should be green. Why?
No need to answer anymore, I fixed it.
1. Self-emission was the culprit. I explored multiple eccentric planets and found out that this issue was not restricted to my planet. It was applicable to all planets. This made it obvious that there was a problem with the game and not the planet. I don't know if self-emission is supposed to work like that, but light emission at such a low temperature (as well as light emission being bypassed on the day side when the planet is close to periastron, even when temperature are above 2000 degrees Celsius) is definitely something that you would encounter in real-life.
2. I changed the atmosphere model from "Jupiter" to "Chlorine" and discovered that the atmosphere was actually pink, explaining this phenomenon. So I changed it to green. What is weird, however, is that the day side atmosphere and the night side atmosphere have different colors (usually complementary colors). The "real" color is the night side color.