Particles Lighting Up at Distance

UE5.1.1 / Lumen / Sky Light - I can’t figure out what threshold this is. Particles are behaving nicely to Lumen within a certain range, but as you can see in the vid, at a certain distance away they light up/emit? Not sure what is going on, anyone familiar with particle lighting that could guide me the right way?

Note The scale for reference, that door is about 20,000 unites across.