This is due to the exposure change, when you go into a dark area the exposure drops but the emissive output stays the same, so it is now relatively brighter than the surroundings and the bloom kicks in and lights it up.
In UE5 we now have the “EyeAdaptationInverse” node in the material graph that will keep the emissive consistent with the exposure. You’ll still see fading saturation at extreme exposures, so you’ll need to be careful of that.
