Using IEEEtran with siunitx's \num{...}
You should use \sisetup
command in your preamble in order to tell siunitx
detect the font weight.
This can be done by using the command:
\sisetup{detect-weight=true,detect-inline-weight=math}
(as suggested here)
Or—as you found out yourself—the command:
\sisetup{detect-all=true}
Full MWE:
\documentclass[conference]{IEEEtran}
\usepackage{siunitx}
\sisetup{detect-all=true}
\begin{document}
\begin{abstract}
foo \num{12.34} 12.34 bar.
\end{abstract}
shit
\end{document}
Or
\documentclass[conference]{IEEEtran}
\usepackage[detect-weight]{siunitx} % <---
\begin{document}
\begin{abstract}
foo \num{12.34}\,bar or \SI[mode=text]{12.34}{bar} % <---
\end{abstract}
\end{document}