Great!
I think I may know what the problem is with that query too. The index table configuration is probably okay, but there is also a limit to ignore single characters within the search query. Can you pelase check minimum word length option and set it to 1 here: https://i.imgur.com/AgQcbLG.png
That should deal with queries like vitamin a
Quoted searches like “vitamin a” may still be an issue because of the index table limitation. The performance benefit comes from indexed queries, but those are only used whenever matches are on the start or the end of the keywords. Basically, when something is placed in quotes like “vitamin a”, it is considered as a single phrase. Then the plugin will try to match “vitamin a” against the start or the end of the database of singular keywords + the post titles. And there is the issue, because “vitamin a” does not actually match any singular keyword nor any title beginning or ending. It would match some post titles within, or whole category names, but remember that is not possible to execute for indexed queries.
I can very likely improve this though. I think if the taxonomy term names would be indexed both as keywords and as whole it would be beneficial, and searches like “vitamin a” would very likely yield results as expected. I will see what I can do about that for todays upcoming release.