I'm not quite sure if this count as a bug. In the following example the definition of testA contains a spurious space. This space deplaces the bounding box of the resulting tikzpicture/plot but curiously only with pdflatex and lualatex. With xelatex both bounding boxes are fine.
\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\begin{document}
\pgfmathdeclarefunction{testA}{1}{%
\pgfmathparse{2} %spurious space
}
\pgfmathdeclarefunction{testB}{1}{%
\pgfmathparse{2}%
}
xx\begin{tikzpicture}
\begin{axis}
\addplot {testA(1)};
\end{axis}
\end{tikzpicture}yy
xx\begin{tikzpicture}
\begin{axis}
\addplot {testB(1)};
\end{axis}
\end{tikzpicture}yy
\end{document}
Fixed in https://sourceforge.net/p/pgf/git/ci/00f4e8d4154dcb3133ed4a106b6254b8faf874e2/ but it broke
gcd. I hope it won't break too much else.As a little gimmick you can get a spaces inside the definition using
~like inexpl3.