feynmp 与 minipages 一起使用时出现奇怪的行为

Question

我正在使用 feynmp-auto 和 subcaption 包。我有一张包含四张相同费曼图的图，使用

\begin{figure}[H] % Do not use only [h] in real documents.
\begin{minipage}[b]{.49\linewidth}
\centering
\begin{fmffile}{feyngraph}
  \begin{fmfgraph*}(150,60)
    \fmfstraight
    \fmfleft{i1,i2}
    \fmfright{o1,h,o2}
    % gluons
    \fmf{gluon}{i1,t1}
    \fmf{gluon}{t2,i2}
    \fmf{phantom,tension=0.5}{t1,o1}
    \fmf{phantom,tension=0.5}{t2,o2}
    \fmffreeze
    % top loop
    \fmf{fermion,tension=1}{t1,t2,t3,t1}
    % Higgs boson
    \fmf{dashes,tension=2.0}{t3,h}
    % labels
    \fmflabel{$g$}{i1}
    \fmflabel{$g$}{i2}
    \fmflabel{X}{h}
  \end{fmfgraph*}
\end{fmffile}
\vspace{4ex}
\end{minipage}
\begin{minipage}[b]{.49\linewidth}
\centering
\begin{fmffile}{feyngraph}
  \begin{fmfgraph*}(150,60)
    \fmfstraight
    \fmfleft{i1,i2}
    \fmfright{o1,h,o2}
    % gluons
    \fmf{gluon}{i1,t1}
    \fmf{gluon}{t2,i2}
    \fmf{phantom,tension=0.5}{t1,o1}
    \fmf{phantom,tension=0.5}{t2,o2}
    \fmffreeze
    % top loop
    \fmf{fermion,tension=1}{t1,t2,t3,t1}
    % Higgs boson
    \fmf{dashes,tension=2.0}{t3,h}
    % labels
    \fmflabel{$g$}{i1}
    \fmflabel{$g$}{i2}
    \fmflabel{X}{h}
  \end{fmfgraph*}
\end{fmffile}
\vspace{4ex}
\end{minipage}
\begin{minipage}[b]{.49\linewidth}
\centering
\begin{fmffile}{feyngraph}
  \begin{fmfgraph*}(150,60)
    \fmfstraight
    \fmfleft{i1,i2}
    \fmfright{o1,h,o2}
    % gluons
    \fmf{gluon}{i1,t1}
    \fmf{gluon}{t2,i2}
    \fmf{phantom,tension=0.5}{t1,o1}
    \fmf{phantom,tension=0.5}{t2,o2}
    \fmffreeze
    % top loop
    \fmf{fermion,tension=1}{t1,t2,t3,t1}
    % Higgs boson
    \fmf{dashes,tension=2.0}{t3,h}
    % labels
    \fmflabel{$g$}{i1}
    \fmflabel{$g$}{i2}
    \fmflabel{X}{h}
  \end{fmfgraph*}
\end{fmffile}
\vspace{4ex}
\end{minipage}
\begin{minipage}[b]{.49\linewidth}
\centering
\begin{fmffile}{feyngraph}
  \begin{fmfgraph*}(150,60)
    \fmfstraight
    \fmfleft{i1,i2}
    \fmfright{o1,h,o2}
    % gluons
    \fmf{gluon}{i1,t1}
    \fmf{gluon}{t2,i2}
    \fmf{phantom,tension=0.5}{t1,o1}
    \fmf{phantom,tension=0.5}{t2,o2}
    \fmffreeze
    % top loop
    \fmf{fermion,tension=1}{t1,t2,t3,t1}
    % Higgs boson
    \fmf{dashes,tension=2.0}{t3,h}
    % labels
    \fmflabel{$g$}{i1}
    \fmflabel{$g$}{i2}
    \fmflabel{X}{h}
  \end{fmfgraph*}
\end{fmffile}  
\vspace{4ex}
\end{minipage}
\caption{A caption.}
\end{figure}

这会产生以下内容（应该如此）：

但是，现在如果我添加一个带有附加费曼图的部分：

\subsection{Test}

\begin{figure}[H]
\centering
\begin{fmffile}{feyngraph}
  \begin{fmfgraph*}(140,90)
    \fmfleft{d,i1,d,d,i3,d}
    \fmfright{o1,d,o2,d,o3}
    \fmf{gluon,tension=1.2}{i1,v1}
    \fmf{gluon,tension=1.2}{v3,i3}
    \fmf{fermion}{o1,v1}
    \fmf{fermion}{v3,o3}
    \fmf{phantom,tension=0.3}{v1,v3}
    \fmffreeze
    \fmf{fermion}{v1,v2,v3}
    \fmf{dashes,tension=1.3}{v2,o2}
    \fmflabel{$g$}{i3}
    \fmflabel{$g$}{i1}
    \fmflabel{$t$}{o3}
    \fmflabel{$\bar{t}$}{o1}
    \fmflabel{H}{o2}
  \end{fmfgraph*}
\end{fmffile}
\caption{test}
\end{figure}

这突然将第一张图的费曼图与第二张图的费曼图切换了：

为什么会发生这种情况？ 有办法防止这种情况发生吗？