Circuitikz wrongfully intersects node

As ever, @js bibra answer is ok. Just to explain a bit more what happens in the following (you can read a quite lengthy and full explication in the manual of circuitikz, section 5.1).

        \draw (0,0) -- ++(0,0.75) to[short,i_=$a_1$] ++(0,0.5) 
        to[short,-o] ++(0,0.5) node[right]{$b_1$} coordinate(b1);

When you use -o, circuitikz will add a node with the shape ocirc to the path. Nodes are drawn after the path is stroked; and the node is designed so that is filled with white. The coordinate b1 is then set at the point where you arrived (the node you added with $b_1$ has not moved it). Now

        \draw [thick, dotted] (b1) -- ++(0,1);

will draw the dotted line starting from b1, which is at the center of the ocirc node.

A different solution is to position the ocirc node after having drawn the dashed line.

\documentclass[border=10pt]{standalone}
\usepackage[siunitx, RPvoltages]{circuitikz}
\begin{document}

\begin{tikzpicture}[arrowmos]
    \draw (0,0) -- ++(0,0.75) to[short,i_=$a_1$] ++(0,0.5) 
        to[short] ++(0,0.5) node[right]{$b_1$} coordinate(b1);
    \draw [thick, dotted] (b1) -- ++(0,1);
    \node [ocirc] at (b1){};
\end{tikzpicture}
\end{document}

(BTW, notice that this is a real MWE).

A very similar problem is explained around page 174, in the FAQ:

Finally, as you comment, if you want to use the north coordinate of the pole, you have to put it explicitly so that you can name it:

\begin{tikzpicture}[arrowmos]
    \draw (0,0) -- ++(0,0.75) to[short,i_=$a_1$] ++(0,0.5) 
        to[short] ++(0,0.5) node[ocirc](b1){} node[right]{$b_1$};
    \draw [thick, dotted] (b1.norht) -- ++(0,1);
\end{tikzpicture}

Moreover, as noticed by John Kormylo, now b1 is a node reference, so the lines going out will start from the border anchor automatically:

\documentclass[border=10pt]{standalone}
\usepackage[siunitx, RPvoltages]{circuitikz}
\begin{document}
\begin{tikzpicture}[arrowmos]
    \draw (0,0) -- ++(0,0.75) to[short,i_=$a_1$] ++(0,0.5) 
        to[short] ++(0,0.5) node[ocirc](b1){} node[right]{$b_1$};
    \draw [thin,  red] (b1) -- ++(0,1);
    \draw [thin, blue] (b1) -- ++(1,1);
    \draw [thin, dashed] (b1) -- ++(-1,1);
    \draw [thin, dotted] (b1) -- ++(0.5,1);
\end{tikzpicture}
\end{document}

\begin{tikzpicture}[arrowmos]
        \draw (0,0) -- ++(0,0.75) to[short,i_=$a_1$] ++(0,0.5) to[short,-o] ++(0,0.5) node[right]{$b_1$} node[inner sep=1pt](b1){};
        \draw [thick, dotted] (b1) -- ++(0,1);
    \end{tikzpicture}