momentum arrow becomes odd when shortened too much

[zhyiyu]

Hi, I am trying to shrink the length of a momentum arrow for a loop, it works fine when the shrink factor is not big, but when it is big, say, 0.7, the momentum arrow is even flipped. For example, the arrow shorten for vertex (b) is where the problem arises

\begin{figure}[H]
\begin{center}
\begin{tikzpicture}
\begin{feynman}
\vertex (a1);
\vertex[right=1.5cm of a1] (a2);
\vertex[right=3cm of a1] (a3);

\vertex[above=1cm of a2] (b);

\diagram*{
(a2) -- [double, out=150, in=180, momentum={[arrow shorten=0.3, arrow style=blue]$k'$}] (b),
(b) -- [double, out=0, in=30, momentum={[arrow shorten=0.7, arrow style=blue]$k'$}] (a2),
(a1) -- [momentum'={[arrow shorten=0.25]$k$}] (a2) -- [momentum'={[arrow shorten=0.25]$k$}] (a3),

};
\end{feynman}
\end{tikzpicture}
\end{center}
\end{figure}

Thanks!