When you specify double, Tikz first draws a line whose width is twice the line width plus the double distance, then draws a white line down the center whose width is the double distance. But the arrowhead is drawn using just the line width, which isn't big enough to cover the ends of the lines.
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows.meta}
\begin{document}
\begin{tikzpicture}
\draw[->,line width=1pt,double distance=1pt] (0,0) -- (1,1);
\draw[->,line width=3pt] (1,0) -- (2,1);
\end{tikzpicture}
\end{document}
Best Answer
Using the arrow tip kind
-Implies
will prevent the shaft from extending beyond the tip, but the white stripe down the center is still apparent.