I’d first check the lumen scene to see what the visibility looks like at various distances, sometimes the object’s lighting appears to be absent but the object itself remains.
Then, I’d turn on lumen.screenprobegather.referencemode. Lumen’s importance sampling lets you get good GI at a low sample count, but it (by design) struggles with small, bright emissives. Reference mode will just disable the optimizations and crank up the ray count massively, so performance will be bad but the emissives should be picked up.