I have the following code. It behaves as expected when only one middle node is drawn; however, when both are there the result is surprising. Why is that? How can I fix it?

```
\documentclass[border=2mm]{standalone}

\usepackage{tikz}
\usetikzlibrary{ calc }
\usetikzlibrary{ graphs, graphs.standard }

\begin{document}
    \begin{tikzpicture}
        \graph[ nodes = { fill = blue }, empty nodes ]{
            subgraph C_n [n = 8, clockwise, radius = 2cm];
            1 -- 9 [at = { ($(1) ! 0.5 ! (6)$) }] -- 6;
            2 -- A [at = { ($(2) ! 0.5 ! (5)$) }] -- 5;
        };
    \end{tikzpicture}
\end{document}

```

[![enter image description here](https://i.stack.imgur.com/oyz1m.png)](https://i.stack.imgur.com/oyz1m.png) [![enter image description here](https://i.stack.imgur.com/Ve4Ab.png)](https://i.stack.imgur.com/Ve4Ab.png) [![enter image description here](https://i.stack.imgur.com/eNuO3.png)](https://i.stack.imgur.com/eNuO3.png)