pgfplots generating images in RGB colorspace no matter what you define

[pauloney]

This seems to be a bug, because you see all over the place recommendations to use

colormap/blackwhite

in the axis properties, to produce an image in the Gray colorspace, as seen https://tex.stackexchange.com/questions/194085/pgfplot-how-to-render-3dplot-with-colors-black-white-and-gray, but it does not work on this example, and it produces elements in RGB colorspace all over the PDF file.

\documentclass{report}
\usepackage{pgfplots}
\pgfplotsset{compat=1.18}

\begin{document}

\begin{tikzpicture}[scale=0.7]
  \begin{axis}[
    domain=0:4,
    samples=30,
    smooth,
    axis lines=center,
    xlabel={$K$}, ylabel={$L$}, zlabel={$f(K,L)$},
    xmin=0, xmax=5, ymin=0, ymax=5, zmin=0, zmax=5, 
    view={80}{30},
    colormap/blackwhite
    ]
    % Define the function to plot
    \def\f(#1,#2){#1^0.5 * #2^0.5}
    \addplot3 [surf] {\f(x,y)};
  \end{axis}
\end{tikzpicture}