With xelatex everything is fine, but with lualatex the second ! is wrong.
up-to-date TL 2016
\documentclass{article}
\usepackage[ngerman]{babel}
\usepackage[autostyle]{csquotes}
\begin{document}
\enquote{foo \enquote{!bar!} baz}
!bar!{}`
\end{document}
Herbert
