In MultiTermHighlighting, a CharacterRunAutomaton is being created that takes the result of AutomatonQuery.getAutomaton that in turn is byte oriented, not character oriented. For ASCII terms, this is safe but it's not for multi-byte characters. This is most likely going to rear it's head with a WildcardQuery, but due to special casing in MultiTermHighlighting, PrefixQuery isn't affected. Nonetheless it'd be nice to get a general fix in so that MultiTermHighlighting can remove special cases for PrefixQuery and TermRangeQuery (both subclass AutomatonQuery).
AFAICT, this bug was likely in the PostingsHighlighter since inception.