Adding a command to beamer's background canvas template shifts the background horizontally

Question

In this answer, @diabonas suggested a way to apply a fixed transition to every slide:

\addtobeamertemplate{background canvas}{\transfade}{}

Why, in this MWE, applying the extension as a pre-text (p.167 of the Beamer manual) results in the background canvas being shifted horizontally? Shifting the \transfade to the post-text magically fixes it.

\documentclass{beamer}
\usepackage{xcolor}
\usepackage{tikz}
\usetikzlibrary{backgrounds, positioning, scopes}
\makeatletter
\definecolor{officialGradient1}{HTML}{223a33}
\definecolor{officialGradient2}{HTML}{0a161f}
\mode<beamer>{
\defbeamertemplate{background canvas}{divoc shading}
{
\pgfuseshading{divoc shading}% simple enough
}
[action]
{
\pgfdeclareverticalshading{divoc shading}{\the\paperwidth}
{color(0cm)=(officialGradient2); color(\the\paperheight)=(officialGradient1)}
}
\setbeamertemplate{background canvas}[divoc shading]
}
\mode<all>
\addtobeamertemplate{background canvas}{\transfade[duration=0.2]}{}
\makeatother
\begin{document}
    \begin{frame}
        \frametitle{Test}
    Test.

\end{frame}

\end{document}