I was able to reproduce it in the Smellenbergh POV-RayUnofficial
MultiPatch for Mac, which is apparently based in part on UVPov 6.1 code.

To the left is a bright beam of light, and off to the right a faint beam
of light continues off into the distance.
It is barely visible, gamma differences may make it invisible on some
systems.
Increasing extinction makes it a lot more visible, extinction of 0 makes
it invisible.
Very strange...

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