I solved this using someone else's advice, found on Reddit:

([\(]) — You need to find a specific character (the opening parenthesis), so you need to enclose it in parentheses. However, because parentheses are special wildcard characters in their own right, you need to tell Word to treat them as normal text characters and not as special characters, so you put in a backslash ‘\‘ (also known as an ‘escape’ character) before the (, AND surround this string in square brackets [ ] (otherwise, it won’t work).