The reason this doesn’t work is because Lumen’s screen traces will pick up anything on screen, even if the mesh is set to not affect distance field lighting. That toggle only works as long as the mesh is not on screen. As noted previously, the only way to avoid this is by using something that Lumen skips screen traces for like translucency.
You can either turn off screen traces for Lumen or set the material to use one of the shading models that skips screen traces (two-sided foliage/hair/subsurface scattering). This may change the lighting response though so you’ll have to weigh that. It should be noted that this will also remove the light from reflections.
Disabling screen traces will reduce the quality of the GI (and potentially increase cost when using HW raytracing), but if you can’t change the blend mode, and you can’t change the shading model… well…