[O] BEAMER_ACT ignored during export in column environment

[Julien Cubizolles]

Consider the following:

--8<---------------cut here---------------start------------->8---
#+STARTUP: beamer
* Frame                                                         :B_fullframe:
:PROPERTIES:
:BEAMER_env: fullframe
:END:
** Column1
:PROPERTIES:
:BEAMER_COL: .45
:END:
1st column
** Column2
:PROPERTIES:
:BEAMER_COL: .45
:END:
*** Column 2 in a heading
:PROPERTIES:
:beamer_act: article
:END:
2nd column
** Column3
:PROPERTIES:
:BEAMER_COL: .45
:beamer_act: article
:END:
3rd column
--8<---------------cut here---------------end--------------->8---

The "article" property of the "Column3" heading isn't exported to the
TeX file whereas the one of "Column 2 in a heading" is. However Beamer
supports this overlay for column environment. In the following
(modified) TeX file, the 3rd column is ignored when processed as it
should be.

Is there a reason why org would choose to ignore overlay specifications
for column environment ?

Julien.

--8<---------------cut here---------------start------------->8---
% Created 2015-09-09 mer. 14:50
\documentclass[presentation]{beamer}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fixltx2e}
\usepackage{graphicx}
\usepackage{grffile}
\usepackage{longtable}
\usepackage{wrapfig}
\usepackage{rotating}
\usepackage[normalem]{ulem}
\usepackage{amsmath}
\usepackage{textcomp}
\usepackage{amssymb}
\usepackage{capt-of}
\usepackage{hyperref}
\usetheme{default}
\author{Julien Cubizolles}
\date{\today}
\title{}
\hypersetup{
 pdfauthor={Julien Cubizolles},
 pdftitle={},
 pdfkeywords={},
 pdfsubject={},
 pdfcreator={Emacs 25.0.50.2 (Org mode 8.3.1)}, 
 pdflang={English}}
\begin{document}

\begin{frame}{Outline}
\tableofcontents
\end{frame}

\begin{frame}[label={sec:orgheadline1}]{}
\begin{columns}
\begin{column}{.45\columnwidth}
1st column
\end{column}
\begin{column}{.45\columnwidth}
\begin{block}<article>{Column 2 in a heading}
2nd column
\end{block}
\end{column}
\begin{column}<article>{.45\columnwidth}
3rd column
\end{column}
\end{columns}
\end{frame}
\end{document}
--8<---------------cut here---------------end--------------->8---