The problem is Unreal’s photo realistic tonemapper being the default for post process, and that’s what blows out the values to white when you push the emissive values too high.
I believe this quick tutorial is still applicable for more saturated Unreal post process. While it’s focused on getting a more diffuse hand painted look instead of realistic lighting, the way it handles color helps keep your emissive from getting blown out as well. https://www.artstation.com/artwork/ybrJgR
There’s also some settings for saturation you can mess with, but overall there are lots of people who’ve had the same issue as you with Unreal and emissive, and there’s no real easy or good fix that doesn’t also change what the rest of the scene looks like.