Avoid bfseries from bolding \pm in siunitx
You can also use the b
font series so the numbers will have the same size.
\documentclass{article}
\usepackage{siunitx,etoolbox,booktabs}
\sisetup{separate-uncertainty}
\begin{document}
\begin{table}
\centering
\renewrobustcmd{\bfseries}{\fontseries{b}\selectfont}
\renewcommand{\pm}{\mathbin{\mbox{\unboldmath$\mathchar"2206$}}}
\begin{tabular}{
c
S[detect-weight, table-format=-1.1(1),mode=text]
}
\toprule
Header1 & {Header2} \\
& {Sub} \\
\midrule
A2 & -6,2 \pm 1,3 \\
A3 & 4,8 \pm 1,7 \\
A7 & \bfseries 5,7 +- 2,0 \\
A12 & 5,9 \pm 2,1 \\
\bottomrule
\end{tabular}
\caption{Caption.}
\label{table:mwe}
\end{table}
\end{document}