D.C.s heading fix for N.L. with two additions by N.L.

\documentclass[paper=a5,pagesize]{scrartcl}
\usepackage{blindtext,xcolor,tikz,eso-pic,titlesec}

\setkomafont{section}{\normalsize}
%-------------------------------------- new
\setkomafont{subsection}{\normalsize}
%-------------------------------------------

% baseline grid
\AddToShipoutPicture{%
\AtTextLowerLeft{
    \begin{tikzpicture}[overlay,remember picture]%
        \draw[red!10!white,thin]%
             (current page.south west) grid[step=\baselineskip]%
             (current page.north east);%
    \end{tikzpicture}%
    }%
}

% spacing for headings
\titlespacing*{\section}{0em}{2\baselineskip}{\baselineskip}
\titlespacing*{\subsection}{0em}{.5\baselineskip}{0.5\baselineskip}
\titlespacing*{\subsubsection}{0em}{\baselineskip}{0em}



%% workaround
%\titlespacing*{\subsection}{0em}{1\baselineskip}{0.5\baselineskip}
%\titleformat{\subsection}{\sectfont{\color{white}\titlerule[0.6\baselineskip]}}{\thesubsection}{1ex}{}

\makeatletter
\def\xsubsection{subsection}
\let\oldttl@straight@ii\ttl@straight@ii
\def\ttl@straight@ii #1#2#3#4#5#6#7{%
\def\tmp{#6}%
\ifx\tmp\xsubsection
\null\nobreak
\fi
\oldttl@straight@ii{#1}{#2}{#3}{#4}{#5}{#6}{#7}}

\begin{document}
\section{Lorem Ipsum}
\Blindtext
\subsection{Lorem Ipsum}
\Blindtext\newpage
{\tracingall
\subsection{Lorem Ipsum}}
\Blindtext
\subsubsection{Lorem Ipsum}
\Blindtext
%-------------------------------------- new
\subsection{Lorem Ipsum}
\Blindtext\Blindtext
\subsection{Lorem Ipsum}
\Blindtext\Blindtext\Blindtext
\subsection{Lorem Ipsum}
\Blindtext\Blindtext\Blindtext\Blindtext
%------------------------------------------
\end{document}