I have a bib entry with a typo at the end of its title. I indicate this with [sic!]. This prevents, however, a period from being printed after the title, cf. the MWE below. Without the exclamation point, a period appears after the title. I assume it has something to do with the exclamation point being interpreted as a symbol ending the title, as you normally wouldn't find the combination !.
anywhere. But here the exclamation point is inside brackets, so it should be followed by a period. I've tried putting the exclamation point or the brackets inside curly braces, but with no effect.
\documentclass{article}
\usepackage[style = authoryear-comp]{biblatex}
\begin{filecontents}[overwrite]{\jobname.bib}
@MVBOOK{unger1877,
TITLE = "Heilagra Manna S{\o}gur",
YEAR = "1877",
EDITOR = "C.R. Unger",
LOCATION = "Christiania",
PUBLISHER = "B.M. Bentzen",
SUBTITLE = "Fort{\ae}llinger og Legender om hellige M{\ae}nd og Kvinder",
TITLEADDON = "Efter gamle Haandsrifter [sic!]",
VOLUMES = "2"}
@MVBOOK{unger1878,
TITLE = "Heilagra Manna S{\o}gur",
YEAR = "1878",
EDITOR = "C.R. Unger",
LOCATION = "Christiania",
PUBLISHER = "B.M. Bentzen",
SUBTITLE = "Fort{\ae}llinger og Legender om hellige M{\ae}nd og Kvinder",
TITLEADDON = "Efter gamle Haandsrifter [sic]",
VOLUMES = "2"}
\end{filecontents}
\addbibresource{\jobname.bib}
\begin{document}
\nocite{unger1877}\nocite{unger1878}
\printbibliography
\end{document}