With \tiny and \scriptsize font sizes, marginnote adds extraneous vertical space after the first line. It seems to work fine with \footnotesize and upwards.
By comparison, \marginpar does not have this issue.
Assuming this is a bug, the question becomes: is there an easy remedy?
\documentclass{article}
\usepackage{marginnote}
\begin{document}
\reversemarginpar
\noindent text\marginnote{\tiny some long text here that includes two line breaks}
\end{document}


\marginfont, but instead just using\marginnote{\strut\parbox[t]{\marginparwidth}{\scriptsize#1}}– steve Oct 10 '21 at 14:16