In a text, I get an overfull
hbox. To avoid this, I define an optional hyphenation point:
bla bla bla un\-known bla bla bla
pdflatex indeed hyphenates the word at that point.
But when I declare the optional hyphenation point in the preamble like this
pdflatex will NOT hyphenate there.
The reason seems to be that LaTeX still thinks that the typeset text does not look good enough with the line break after 'un-'.
But then why is the behavior different for the inline optional hyphenation to the optional hyphenation declared in the preamble?