Tikzset 无故修改图片

Question

由于某种原因，tikzset第一次和第二次绘制的图片似乎有所不同。在下面的例子中，我希望我绘制的两个神经网络是相同的，其中一个直接位于另一个的右侧：

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usetikzlibrary{calc}
\usetikzlibrary{shapes.geometric}
\usetikzlibrary{fit}
\usetikzlibrary{positioning}
\usetikzlibrary{backgrounds}


\begin{document}
\begin{tikzpicture}
    \tikzset{    
        box/.style = {
            draw, 
            minimum width=3em, 
            minimum height=3em
        },
        nn/.style = {
            draw, 
            circle,
            thick,
            black,
            inner sep=0pt,
            text width=1mm,
        },
        edge/.style = {
            -,
            black,
            semithick
        },
        mlp/.pic= {%
            \node[nn] (n0) {};
            \node[nn, below=0.2em of n0] (n1) {};
            \node[nn, below=0.2em of n1] (n2) {};
            
            \node[nn] (n3) at ([xshift=0.6em]$(n0)!0.5!(n1)$) {};
            \node[nn] (n4) at ([xshift=0.6em]$(n1)!0.5!(n2)$) {};
            
            \draw[edge] (n0.east) -- (n3.west);
            \draw[edge] (n1.east) -- (n3.west);
            \draw[edge] (n2.east) -- (n3.west);
            
            \draw[edge] (n0.east) -- (n4.west);
            \draw[edge] (n1.east) -- (n4.west);
            \draw[edge] (n2.east) -- (n4.west);
            
            \begin{scope}[on background layer]
                \node[draw, fit=(n0) (n2) (n4), rounded corners=5pt, inner sep=2pt, thick] (box) {};
            \end{scope}
        },
    }
    \pic[](mlp0_){mlp};
    \pic[right=of mlp0_box](mlp1_) {mlp};
\end{tikzpicture}
\end{document}

相反，第二个框向下移动，并且比第一个框更宽。

为什么是这样？