This is the already-known issue of lights appearing black with directionality (IES is a form of directionality - the file specifies the intensity of the light in particular directions). "Avoid black appearance" only works for direct line of sight to the light source. Current workaround is to use two lights, one visible to the camera with no directionality (no IES) and not emitting light, one invisible to the camera with directionality (IES) and emitting light.
