Sorry I didn't realize that Turner released a new version 5.24
which is supposed to support conversion from %mn to a multibyte
character set. As he mentioned in his release note, it didn't mean
that this version with "SEARCHCHARCONVERT ON" can reproduce
multibyte characters correctly. And you showed that it realy didn't
reproduce Japanese characters correctly.
urldecode.pl cannot be applied to this HTML output since it meant
to convert %mn to a multibyte character set as you noticed already.