As can be seen in http://tex.stackexchange.com/questions/224196/problem-with-in-to-path-angle-when-negative-scale-factor-are-applied when a negative value is used for {x|y}scale. I think the problem is also true for positive factors, but most evident with negative ones.
The input
angle is not the incidence angle to node's center, but also the real anchor. When scale is negative, the factor is not applied to input
angle while it's to output one
.
A little example with its result attached:
\begin{tikzpicture}[thin,>=stealth, every place/.style={draw,thick,minimum size=6mm}]
\node[place] (pG) {$p_G$};
\node[place,right=3cm of pG] (pR) {$p_R$};
\draw[->] (pR) to [out=150, in=30] (pG);
\begin{scope}[yscale=-1]
\draw[->,blue] (pR) to [out=150, in=30] (pG);
\end{scope}
\begin{scope}[xscale=-1]
\draw[->,red] (pR) to [out=150, in=30] (pG);
\end{scope}
\begin{scope}[scale=-1]
\draw[->,green] (pR) to [out=150, in=30] (pG);
\end{scope}
\end{tikzpicture}
Closing issue since the tex.sx thread comes to the conclusion that it works as designed.