the & you see is not really there in the relational DB.
The DB has 1 entry per author. each has an index, the book has a list of thoe indexes for authors (this is why you can fix the text of Author in the Tag browser and ALL uses get the update. You fixed the Author table, not the book)

Check in the Tag browser: Authors, to see if you have variants of the authors name (case, initials, spacing)


Get the Quality check Plugin: It can find and help Fix(make consistent) many metadata format items listed above