This was simply a bug, I fixed it, thanks for the report.
Depends on the light paths involved in the scene.
Bidir has the problem that it can’t render SDS paths (specular-diffuse-specular, for example caustics on the bottom of a pool seen from above the water surface). Our unidirectional Path engine, on the other hand, can render SDS paths because it can use the PhotonGI caustics cache. In such a scene, Bidir would actually be unable to produce the correct result.
I think since Path has been augmented with the “add light tracing” feature and the caustics cache, there are only a few scenarios left where Bidir is more correct.