Why is `default preamble' in `/forestset` ignored?

Question

\documentclass[oneside,12pt]{article}
\usepackage{geometry}
\usepackage{microtype}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}

\usepackage{enumitem}
\setlist[enumerate,1]{label=\bfseries\Alph*,align=left,leftmargin=*,
  labelsep=1.5em}
\setlist[enumerate,2]{label=\arabic*.,labelindent=1em,labelsep=1.5em,
  leftmargin=*}

\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{amsthm}
\theoremstyle{definition}
\newtheorem*{sol}{Solution}

\usepackage{forest}
\forestset{
  ass/.append style={
    before computing xy={l=\baselineskip},
    no edge
  },
  prooftree/.style={
    for tree={
      child anchor=north,
      parent anchor=south
    }
  },
  default preamble={
    prooftree,
    for root={baseline},
    for tree={%
      if n children={1}{for children={ass}}{},
      math content
    }
  }
}

\begin{document}
\subsection*{Exercises 28}
\begin{enumerate}
\item Let's use `$\vDash$' now to abbreviate `q-entails'. Which of the
  following claims are true? Provide arguments in the style of 28.2 and 28.6
  to defend your answers.
  \begin{enumerate}
  \item $\forall x(Fx\supset Gx)\vDash\forall x(Gx\supset Fx)$
    \begin{sol}
      Try the following q-valuation:
      \begin{quote}
        Domain = all people\\
        Extension of `F' = all singers\\
        Extension of `G' = all people who know how to sing
      \end{quote}
      Then, the premiss is true, but the conclusion is false.
    \end{sol}
  \item $\forall x(Fx\supset Gx)\vDash\forall x(\neg Gx\supset\neg Fx)$
    \begin{sol}
      \begin{forest}
        [$\forall x(Fx\supset Gx)$
        [$\forall x$]]
      \end{forest}
    \end{sol}
  \end{enumerate}
\end{enumerate}
\end{document}

produces

But, I expected to see.

How can I have default preamble applied to forest environment?

Answer 1

You are using version 1 of Forest. default preamble is provided in version 2. To make this work you must:

1. update Forest to version 2 (preferably 2.0.1, preferably by updating your TeX distribution);

2. remove the $ signs from the final tree.

   \begin{forest}        
     [\forall x(Fx\supset Gx)
       [\forall x]
     ]
   \end{forest}
   

You cannot use default preamble if you have access to only version 1 of the package. If I revert to version 1 and replace the $ signs, then I get the same output as you.