Translucent materials can’t use Lumen’s screen probes because they’re placed directly on the depth buffer, but Translucent materials don’t write depth. So they fall back to being lit by Lumen’s translucent volume lighting, which is very sparse and prone to light leaking. (If I recall correctly the Lumen team originally warned that you’d see leaking on translucent surfaces within 2 meters of any wall)… At least this is my understanding of it.
When you’re using hit lighting for reflections it will usually only be an issue for diffuse lighting, which is why setting the base color to zero hides it, since the indirect specular is contributing most/all the lighting.
A while back in the Lumen feedback thread it was mentioned that the Lumen team was planning (or considering?) leveraging the fact that the translucent probes really only need to be in the camera frustum and not everywhere in worldspace, allocating most of the resolution toward what’s in frame would increase quality which should help address the leaking, but I’m not sure if this is something they’ve started work on yet.