documentclass{article}
usepackage{tikz}
begin{document}
begin{tikzpicture}
    % Here the outer node is simply drawn as a rectangle, the inner tikz is drawn as expected.
    node[draw] at (0,0) {This is a Node with a tikzdraw (0,0) rectangle (.5,.5); rectangle inside.};

    % Here the outer node is correctly drawn with rounded corners.
    % I would expect the inner tikz to render the same as above, but it inherits the rounded corners as well
    node[draw, rounded corners=1ex] at (0,-2) {This is a Node with a tikzdraw (0,0) rectangle (.5,.5); rectangle inside.};

    % I do not observe this behaviour with other style attributes.
    % Below the inner tikz is black with normal line width.
    node[draw=green, very thick] at (0,-4) {This is a Node with a tikzdraw (0,0) rectangle (.5,.5); rectangle inside.};
end{tikzpicture}
end{document}