I was able to reproduce the bug on level 24 as well, with some tricks.

Change the light properties of the level to ambient 12 and bonus 9.

Place a sign somewhere.
Place another obstacle which  (lamp for example) nearby.

Since only obstacles near tux are computed, this bug will only be visible within
certain range near the sign.

You must be in eye-contact with the sign. Otherwise there will be no bug.

When you now are near the sign, there is no other obstacle blocking the path
('view') and you are in the light radius of another obstacle, this black aura
will appear around tux.
As soon as the sign obstacle isn't computed anymore, the light radius will be
back to normal-