If I understand correctly how Relevanssi highlights excerpts, I can only get excerpt highlights if I check both Custom Excerpts and Show Breakdown.
I realize, for an excerpt generated by WP or a custom excerpt, that the excerpt might or might not contain the search key word. However, even if the key word is present in such an excerpt, Relevanssi does not highlight it.
I suggest: Have Relevanssi highlight the key word in an excerpt generated by WP or custom if the term is in that excerpt. (And if the key word is not in the excerpt, then nothing is highlighted.)
Thanks. --Bill