Lights over custom GP

Sorry, I haven't the slightest idea . It does not happen here so there is no way to help you out unless you have some material settings that are different that would influence the dawn/dusk LM texture. But that would be a first I hear of.

Please try:
Assume Vertical Normal: True
Double Sided: True
No Shadow: True

Destination Blend: One
Source Blend: one

Emissive Mode: AdditiveNightOnly