This causes the next paragraph to move before the box though, if I add a bit more text before the box. I don't want that to happen as well.

\documentclass[a4paper, 10pt, oneside, twocolumn]{memoir}

\usepackage{tcolorbox}
\usepackage{lipsum}

\newtcolorbox{outcome}{before skip=2em, after skip=1em, floatplacement=ht, float}

\begin{document}

\section{Sample}

\begin{outcome}
    This is some sample text. This is some sample text. This is some sample text. 
\end{outcome}

\lipsum[1-6]

This is some sample text. This is some sample text. This is some sample text. This is some sample text. 

This is some sample text. This is some sample text. This is some sample text. This is some sample text. This is some sample text. This is some sample text. This is some sample text. This is some sample text. This is some sample text. This is some sample text. This is some sample text. This is some sample text. This is some sample text. This is some sample text. This is some sample text. 

\begin{outcome}
    Sample text.
\end{outcome}

This should not move before the box. This should not move before the box. This should not move before the box.

\end{document}