InnoSetup exists in Unicode and non-Unicode flavors (since 5.3, and latest release as of today is 5.5.5).
The packager checks presence and version of InnoSetup in the EXE case but doesn't check if it is the Unicode version or not.
The iss file created by the packager is written using ASCII-7, which means any parameter value containing say an accented character will be altered.
Ideally the packager would use UTF-8 and recognize as valid the Unicode flavor of InnoSetup and only this one.