titlesec: loss of section numbering with the new update (2016/03/15)

Update

The issue has been solved with titlesec version 2.10.2 (released 2016/03/21)

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Original answer

The bug in the new version 2.10.1 is not properly starting horizontal mode as it was done previously.

You can cure the bug by adding

\usepackage{etoolbox}

\makeatletter
\patchcmd{\ttlh@hang}{\parindent\z@}{\parindent\z@\leavevmode}{}{}
\patchcmd{\ttlh@hang}{\noindent}{}{}{}
\makeatother

after \usepackage{titlesec}.

Here is the “proper” code for \ttlh@hang (titlesec.sty line 746)

\def\ttlh@hang#1#2#3#4#5#6#7#8{%
  \gdef\ttl@makeline##1{\ttl@calc\hspace{#6}##1\ttl@calc\hspace{#7}}%
  \setlength\leftskip{#6}%
  \setlength\rightskip{#7}%
  \interlinepenalty\@M
  \ttl@changecentercr
  \ttl@beginlongest
  #1{\ifhmode\ttl@hmode@error\fi
     \ttl@glcmds
     \parindent\z@
     \leavevmode % <------ MISSING
     \begingroup
       \ifttl@label
          \sbox\z@{#2\strut\ttl@calc\hspace{#3}}%
          \hangindent\wd\z@
          \box\z@ % \noindent was redundant
       \fi
       #4{#8}%
       \kern\z@\strut\@@par
     \endgroup
     \nobreak\ttl@midlongest#5\@@par}%
  \ttl@endlongest}

For Ubuntu 16.04 with texlive:

sudo wget http://mirrors.ctan.org/macros/latex/contrib/titlesec/titlesec.sty -O /usr/share/texlive/texmf-dist/tex/latex/titlesec/titlesec.sty

For other Linux distros:

First run locate /titlesec.sty then adapt the previous code.

Local tree for a single non-admin user:

mkdir -p ~/texmf/tex/latex/titlesec/ ; wget http://mirrors.ctan.org/macros/latex/contrib/titlesec/titlesec.sty -O ~/texmf/tex/latex/titlesec/titlesec.sty