Explain the problem.

Pauses (. . .) between columns are not interpreted as such when converting to revealjs format:

::: columns
:::: column
Left column
::::

. . .

:::: column
Right column
::::
:::

...becomes...

<section class="slide level6">

<div class="columns">
<div class="column">
<p>Left column</p>
</div><p>. . .</p><div class="column">
<p>Right column</p>
</div>
</div>
</section>

Compare this to the beamer output:

\begin{frame}
\begin{columns}[T]
\begin{column}{0.48\textwidth}
Left column
\end{column}

\pause

\begin{column}{0.48\textwidth}
Right column
\end{column}
\end{columns}
\end{frame}

Pandoc version?

pandoc 2.14.2 on Ubuntu 18.04.5 LTS