Search term in middle of word not highlighted by relevanssi_the_title

That is correct. The middle-of-the-word searching only applies to search queries, it has no effect on highlighting. There’s currently no way to change that; middle-of-the-word highlighting is not available, because with short words, it creates very messy results.

I can throw in a filter hook in the next version so that middle-of-the-word highlights can be enabled that way.

If you want a hotfix, modify /lib/excerpts-highlights.php and add

$regex = apply_filters( 'relevanssi_highlight_regex', $regex, $pr_term );

before this line:

$content .= ' ';

Then you can use this filter hook to modify the regex:

add_filter( 'relevanssi_highlight_regex', function( $regex, $pr_term ) {
  return "/($pr_term)/iu";
}, 10, 2 );