Hello,
I'd go even a little further : assuming it makes sense to underline
within a section header, does it also make sense to propagate that
formatting to the toc and/or header of the document ? If not, I suggest
exporting as:
\section[not-so-plain text]{\uline{not-so-plain} \textbf{text}}
Also I don't know why there was underlining in the first place. If we're
asking to underline all section titles, then it might make sense to
redefine e.g. \section to do so (packages titlesec might help).