No hyphenation of long words in narrow boxes

Question

I stumbled across an hyphenation problem which I cannot explain or solve. When I use a narrow box containing a long word with a width longer than the box width, this word is not hyphenated.

The following MWE uses the long word Rechnungsadjunktentochter with an explicit hyphenation pattern in the preamble. This word is hyphenated in a long text, also when the break points are given in a narrow box locally, but it is not hyphenated automatically.

What happens here? And, more important, how can I get my words hyphenated automatically as expected?

UPDATE: @egregs comments explain what happens. But, still, I would have to insert \hspace{0pt} at every first word of every paragraph of every narrow box (and I have MANY of them, partly automatically generated). How can I force TeX to hyphenate the first word of EVERY paragraph (inside an environment) automatically?

UPDATE 2: The solving answer is given by the latter comments of @egreg to this question.

\documentclass{article}

\hyphenation{%
  Rech-nungs-ad-junk-ten-toch-ter
}

\begin{document}

\textit{Works even without the special hyphenation pattern:}\\
bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla Rechnungsadjunktentochter

\bigskip
\textit{Local hyphenation pattern works as expected:}\\
\fbox{\begin{minipage}{3cm}
Rech\-nungs\-ad\-junk\-ten\-toch\-ter
\end{minipage}}

\bigskip
\textit{\bfseries Hyphenation pattern from the preamble does not work as expected:}\\
\fbox{\begin{minipage}{3cm}
Rechnungsadjunktentochter
\end{minipage}}

\end{document}