We can't do the optimization to shorten the ray when we might still need
to go through transparent surfaces or volumes to reach the light.
This issue was not light tree specific, however in the test file it was
more noticable because the light tree poorly handles some areas. This in
in turn causes MIS weights for forward path tracing to become higher,
which is where the error was.
Pull Request: https://projects.blender.org/blender/blender/pulls/122404
5d213d78e3