Dots don't appear in vertices

[mrnovoa]

I'm trying to add red dots in the vertices but, for a certain reason they don't appear. I'm compiling with lualatex on overleaf.

\documentclass[a4paper,12pt]{article}
\usepackage[compat=1.1.0]{tikz-feynman}

\begin{document}

\begin{tikzpicture}
\draw [decoration={markings,mark=at position 1 with
    {\arrow[scale=3,>=stealth]{>}}},postaction={decorate}] (5,1) -- (6.5,1);
\begin{feynman}
\vertex (a1) {\( b\)};
\vertex[right=1.5cm of a1, dot, red] (a2);
\vertex[above right=1cm of a2] (a3);
\vertex[below right=1cm of a3, dot, red] (a4);
\vertex[right=1cm of a4] (a5) {\( s\)};
\vertex[above=1cm of a5] (c1) {\(\ell ^+\)};
\vertex[above=1cm of c1] (c3) {\(\ell ^-\)};
\vertex[dot, red] at ($(c1)!0.5!(c3) - (1.5cm, 0)$) (c2);
%\vertex[above=1.5cm of a5] (c2);
\vertex[right=3cm of a5] (a1p) {\( b\)};
\vertex[right=2cm of a1p, dot, red] (a2p);
\vertex[right=2cm of a2p] (a5p) {\( s\)};
\vertex[above=1cm of a5p] (c1p) {\(\ell ^+\)};
\vertex[above=1cm of c1p] (c3p) {\(\ell ^-\)};
%\vertex[above=1.5cm of a5] (c2);

\diagram* {
{
{[edges=fermion]
(a1) -- (a2) -- [quarter left,edge label=\(t\),green!50!black](a3) -- [quarter left,edge label=\(t\),green!50!black] (a4) -- (a5)},
},
(c3) -- [fermion] (c2) -- [fermion] (c1),
(a3) -- [photon, bend left,edge label=\(Z\),green!50!black] (c2),
(a2) -- [boson, out=-60, in=240, looseness=1.50, edge label'=\(W^{+}\),green!50!black] (a4),
{
{[edges=fermion]
(a1p) -- (a2p) --  (a5p)},
},
(c3p) -- [fermion] (a2p) -- [fermion] (c1p)

};

\end{feynman}

\end{tikzpicture}
\end{document}

[martingabelmann]

For those who also struggle with this: you need to prodive (empty) node content to the vertex, otherwise its still zero-sized (according to the docs):

\vertex[dot](a){}

worked for me.