MakeUppercase not recognizing ß

[Martin Münch]

When lualatex-ing this code:

\documentclass{article}
%\usepackage{libertine}
%\usepackage{libertinus}
%\usepackage{ebgaramond}
\begin{document}
\ss\ \SS\ ß\ ẞ

\MakeUppercase{\ss\ \SS\ ß\ ẞ}
\end{document}

I get a lot of "SS", which I understand, because the capital ẞ is 
missing in the font. Now with any of the three packages, ẞ is available.

But: \MakeUppercase{ß} still produces "SS" instead of "ẞ", which 
everything in \MakeUppercase{\ss\ \SS\ ẞ} produces.

On the one hand just writing \ss instead of ß solves the immediate 
issue, but on the other hand UTF-8 is supported and should also work for 
ß, shouldn't it?