float placement bottom of subcaption on separate page

I used the subcaption package in the past to format two or more images next to each other in a single figure float environment and it worked as expected. Now, I try to use it again but cannot get a placement of the subcaptionbox at the bottom of the first page.

\documentclass{scrreprt}

\usepackage{graphicx}
\usepackage{subcaption}

\usepackage{lipsum}

\listfiles

\begin{document}

% \begin{figure}[b]
%   \centering
%   \includegraphics[width=0.49\linewidth]{example-image-a}
%   \caption{single}\label{fig:plane}
% \end{figure}

\begin{figure}[b]
  \centering
  \subcaptionbox{first\label{fig:first}}
  [0.49\linewidth]{\includegraphics[width=\linewidth]{example-image-a}}
  %
  \subcaptionbox{second\label{fig:sec}}
  [0.49\linewidth]{\includegraphics[width=\linewidth]{example-image-a}}
  %
  \caption{subcaption test}\label{fig:subtest}
\end{figure}

\lipsum[1-6]

\end{document}

Instead of the bottom, the float is shifted to an own [p] as it seems:

With a single image only it works (commented part) or with a [!b] enforcing the position.

How can I get again a placement at the bottom of the first page as in the single image example?